TEX82.DIF[TEX,DEK]8 - www.SailDart.org

perm filename TEX82.DIF[TEX,DEK]8 blob sn#721614 filedate 1983-08-01 generic text, type C, neo UTF8

COMMENT ⊗ VALID 00035 PAGES
C REC PAGE DESCRIPTION
C00001 00001
C00006 00002 This file describes differences between TeX82, which is the portable standard
C00011 00003 * Characters by themselves can no longer masquerade as numbers. Thus, you
C00015 00004 * New input capabilities greatly expand the class of potential applications:
C00019 00005 * \chpar has been abolished. In its place, all of the integer parameters have
C00023 00006 * The \tracing parameter disappears, and its various components each have their
C00028 00007 * An array of 256 dimension values is introduced, called \dimen0 to \dimen255.
C00031 00008 * If you put \global in front of \def or \let or \catcode or \tolerance or
C00035 00009 * If you say \message{text}, the terminal will display " text" immediately. For
C00040 00010 * The internal character set used by TeX82 is the same regardless of the
C00050 00011 * Note that the backslash character is now predefined as an escape character
C00056 00012 * \indent takes you from vertical mode to horizontal mode and indents the
C00061 00013 * Up to 256 fonts may be used, and each font may contain up to 256 characters.
C00066 00014 * TeX82 has a new "help" facility available on error messages. If you type "h"
C00071 00015 * The \mathcode now has a 15-bit number as its value. The first three bits
C00079 00016 * The spacing in math formulas is unbundled too there are three parameters
C00084 00017 * The maximum penalty has been raised from 1000 to 10000 thus, "\penalty 1000"
C00089 00018 * Major changes have been made to the \output conventions, so that it will be
C00104 00019 * In vertical mode, you can say \prevdepth=3pt to make TeX82 act as if the
C00109 00020 * Characters catcoded 13 are not equivalent to one-letter control
C00114 00021 * \hbox par is eliminated! Instead of "\hbox par 100pt{...}", one now says
C00119 00022 * \accent is allowed only in horizontal mode \mathaccent only in math mode.
C00126 00023 * Here is an extension to the language intended to placate people who
C00130 00024 * Syntactic conditionals!
C00134 00025 * \everymath{...} inserts its tokens into TeX's scanner just when
C00137 00026 * \hangindent <dimen> no longer looks for keywords "for" and "after" it's
C00142 00027 * New integer parameter \globaldefs (is normally zero). If positive, all
C00148 00028 (the following changes have been introduced in version 0.96)
C00154 00029 Some new macros of PLAIN.TEX
C00156 00030 (the following changes have been introduced in version 0.97)
C00160 00031 (updates to the April 1983 manual)
C00171 00032 Changes to PLAIN.TEX conventions since the April manual:
C00174 00033 (changes installed in version 0.99)
C00177 00034 Version 0.999
C00185 00035 Changes after July version of TeXbook
C00186 ENDMK
C⊗;
This file describes differences between TeX82, which is the portable standard
definition of TeX in PASCAL, and the TeX systems written in SAIL. (The SAIL
version will NOT be brought up to date to make it compatible with TeX82; its use
should gradually die away as more people take advantage of the new features
available in the PASCAL version.)

* TeX82 does all its calculations that affect line breaking and page breaking
using fixed-point integer arithmetic of limited (i.e., 32-bit) precision,
instead of with floating-point computations, since different machines differ so
widely in the results you get with floating point.

Dimensions are integers in units of 2↑(-16) points, limited in magnitude to 2↑14
points (which is 18.89 feet). This applies to all dimensions (e.g., the heights
and widths and depths of boxes, the amounts by which you \raise or \lower a box,
\varunits, etc.), except for the dimensions of stretching and shrinking.

Something different had to be done with respect to the dimensions of stretching
and shrinking, since for example the old TeX defined \hfill to be a stretch of
10↑10 points, and that number has more than 32 bits to the left of its binary
point. After considering various alternatives, the solution introduced by the
designers of MESA-TeX in 1979 has been adopted for TeX82. Each stretch or shrink
dimension is specified by a fixed point integer that is either in units of
2↑(-16)pt or 2↑(-16)fil or 2↑(-16)fill or 2↑(-16)filll. Here pt is, of course,
one point; the other units are three orders of infinity, essentially infinity
and infinity↑2 and infinity↑3. To add together such units of stretching or
shrinking, one simply adds the individual components having the same order of
infinity, and then uses the nonzero component having the highest order.

Thus, when one says "\hskip <a>pt plus fil minus <c>filll", the numbers <a>,
, <c> are rounded to the nearest multiple of 2↑(-16), and their magnitudes
should be less than 2↑(14). The stretch component is times infinity, and the
shrink component is <c> times infinity cubed.

* The shrink component of all glue used in a paragraph should be finite.
Something like "\hskip 20pt minus 2fil" actually makes no sense in a paragraph,
since the paragraph would fit on a single line no matter what. Infinite
shrinkage does make sense in a simple \hbox, of course.

* Identifiers for control sequences in TeX82 are letter strings of any length,
with upper and lower case letters treated as distinct even when they aren't the
first letter. For example, \TeX is not the same as \TEX, and \GAMMA is not the
same as \Gamma. Any character that is regarded as a letter (this means the 52
letters, initially, plus others that are \chcoded to 11) can appear in such
control sequences. Of course, the one-character non-letter control sequences
still exist as well.
* Characters by themselves can no longer masquerade as numbers. Thus, you
shouldn't say "\chcode A" any more; this has caused more mysterious errors than
it was worth, and anyway there will be a macro for defining fonts symbolically
in TeX82. Instead, one can use a left quote in front of a character to get its
ascii code; e.g., `A is '101, and `↑↑A is 1. Furthermore you can use an escape
in front of the character, e.g., `\A or `\% (these macros will not be expanded),
so that we avoid the problem in TeX80 that you couldn't say "\chcode %" when %
had already been chcoded.

* Hexadecimal constants are allowed, using a prefixed ". For example, "DF = '337
= 223.

* \uccode <char number> is the character code to use when converting to upper
case in the \uppercase function. For example, \uccode`a =`A. (Probably nobody
will change the default values unless a foreign alphabet is being used.)
Characters whose uccode is zero will not be changed by the \uppercase function.

* \lccode <char number> is the character code to use when converting to lower
case in the \lowercase function or when trying to hyphenate a word that contains
upper case letters. For example, \lccode`A =`a. Characters whose lccode is zero
will not be changed by the \lowercase function. The lccode is also used for
hyphenation; a character whose lccode is zero will be regarded as a nonletter
(e.g., punctuation), and not part of a hyphenatable word, while a character
whose lccode is identical to the character itself will be considered a lower
case letter. For example, \lccode`a =`a.

* If \uchyph is nonzero, a word whose first letter is upper case will be subject
to hyphenation. (This means a word whose first letter has lccode nonzero and
lccode not equal to the character itself.) Words whose first letter is lower
case will always be subject to hyphenation even if they contain upper case
letters further on.

* Both \looseness and \parshape are now reset to their default values after each
paragraph, as \hangindent always was. They are also reset at the beginning
of a \vbox.

* Font codes are now control sequences instead of letters, so there is
no longer a 64-font restriction. For example, one says "\font\ff=cmr10 at 12pt"
to load the font identified by \ff. A subsequent appearance of the control
sequence \ff will select this font. (The \: control sequence is no longer
present in TeX82.) Font control sequence names must have length 2 or more.
* New input capabilities greatly expand the class of potential applications:

\openin n=filename, where the = is optional and where the stream number n
is between 0 and 15, designates a text file that can be read
concurrently with other input. If a file has already been opened for
the same number n, it is closed first. If the file doesn't exist, no
file will be opened.

\closein n simply closes stream n.

\ifeof n ... \else ... \fi tests if input stream n is either not open
or has been fully read.

\read n to\cs and \global\read n to\cs: These are the important new commands.
They define control sequence \cs to be the contents of the next line from
input stream n, reading that line in the normal way: the current catcodes
are used, spaces are ignored at the beginning of the line, a blank line
comes through as \par, etc. If input stream n has not been opened, or if
an opened file has been fully input, or if n is negative or greater than 15,
a line is read from the terminal instead; the user is prompted "\cs=" in
this case. The latter feature allows convenient interactive routines, e.g.,
\message{Please type your name:}
\read-1 to\myname
\message{Hello, \myname!}

* The previous commands \open and \send are renamed \openout and \write,
to show their similarity to \openin and \read.

\closeout <number> will close a file so that your TeX program can eventually
input it. (Previously you could only do this by, e.g., \openout 0=empty0.tmp,
cluttering up the disk with an empty file.)

\write, \openout, and \closeout will be ignored if they occur within a \leaders
construction. (Reason: The number of times a leader box occurs might be 0 or 1
depending on floating-point rounding. This restriction keeps the language
independent of floating point.)

* \ifx now allows comparison of any two tokens, not just control sequences.
A non-control-sequence matches only another non-control-sequence that
has the same catcode and represents the same character.

* New feature \expandafter t, where t is any token (usually a control sequence):
If the token following t is a macro, it is expanded as if t were not there.
Then t is put back in front of the result. For example,
\def\a{\x\y\z} \def\b#1\z{#1} \expandafter\b\a
yields \b\x\y\z which yields \x\y. (It used to be possible to do this only
with trickery/hackery.)
* \chpar has been abolished. In its place, all of the integer parameters have
names instead of numbers. (Thus at last they become consistent with the
dimension parameters \hsize, etc., and with the glue parameters \baselineskip,
etc. My only excuse for bad design in the first place was that the integer
parameters were afterthoughts, stuck in after TeX was first up and running; it
was the easiest way to vary some of the originally fixed constants. I wanted to
finish TeX in a year and get on to writing Volume 4! That is still my wish.)

Here are the names of the integer parameters:
\tolerance (formerly \chpar1=) badness tolerance after hyphenation
\pretolerance (formerly \chpar15=) badness tolerance before hyphenation
\hyphenpenalty (formerly \chpar2=) hyphenation penalty
\finalhyphendemerits penultimate line hyphenation demerits
(formerly half of \chpar3=)
\doublehyphendemerits double-hyphen demerits
(formerly half of \chpar3=)
\widowpenalty (formerly \chpar4=) widow line penalty
\brokenpenalty (formerly \chpar5=) broken line at page end penalty
\binoppenalty (formerly \chpar6=) math binary op break penalty
\relpenalty (formerly \chpar7=) math relation break penalty
\predisplaypenalty (formerly \chpar9=) penalty for breaking before a display
\mag (formerly \chpar12=) 1000 x magnification ratio
\adjdemerits (formerly \chpar13=) adjacent incompatibility demerits
\looseness (formerly \chpar14=) change in paragraph length
\uchyph (formerly \chpar16=) uppercase hyphenation
\exhyphenpenalty (formerly \chpar17=) explicit hyphenation penalty
\day (new) initialized to current day of month
\month (new) initialized to current month of year
\year (new) initialized to current year
\time (new) initialized to minutes since midnight
\interlinepenalty (new) see below
\postdisplaypenalty (new) see below
\displaywidowpenalty (new) see below

The former \chpar10 (dump window) is no longer needed, since TeX82 has better
ways to display token lists. The former \chpar11 (\radsign) goes away in favor
of the new \radical primitive. The former \chpar18, \chpar19, \chpar20, once
"reserved for extensions", are gone too, since it is now best for a TeX extender
to give names to whatever new parameters are needed. Similarly, \x is gone.
* The \tracing parameter disappears, and its various components each have their
own names. They are as follows:

\showboxbreadth (nodes per level when a box is being exhibited)
\showboxdepth (maximum level shown when a box is being exhibited)
\pausing (if nonzero, lines from a file are displayed as they appear)
\tracingonline (if nonzero, diagnostic info goes to terminal as well as to file)
\tracingmacros (if nonzero, shows macros as they are being expanded)
\tracingstats (if nonzero, shows memory usage when TeX has recorded it)
\tracingoutput (if nonzero, shows boxes when they are shipped out)
\tracinglostchars (if nonzero, shows chars dropped because they aren't in font)
\hfuzz (a dimen parameter; hboxes are reported if more overfull than this)
\vfuzz (a dimen parameter; vboxes are reported if more overfull than this)
\hbadness (under/overfull hboxes exceeding this (integer) badness are reported)
\vbadness (under/overfull vboxes exceeding this (integer) badness are reported)

* The new \tolerance and \pretolerance are devalued by a factor of 100 from the
old \jpar and \jjpar. In other words, the default is now \tolerance 200 and
\pretolerance 200, so that it is a true "badness tolerance", i.e., the badness
should not exceed 200. Any value of 10000 or more is equivalent to an infinite
value, in which glue can stretch arbitrarily far.

* Furthermore, \codeval and \parval are eliminated. In their place is a much
more powerful operator called \the. For example, what used to be "\codeval5" is
now "\the\catcode5"; what used to be "\parval2" is now "\the\hyphenpenalty". You
can even say "\the\hsize" to get the current \hsize as a dimension,
"\the\baselineskip" to get the current \baselineskip as a glue value; and
you can say things like "\vbox to \the\baselineskip", which makes a vbox
whose height is the normal amount of baselineskip (exclusive of stretching
and shrinking). When expanding macros, you can say "\the\font" to get the
current font identifier (e.g., "\ff"), as well as "\the\ff" to get the
corresponding font name, as well as "\the\output" and "\the\everypar".
In the latter cases, macros inside the current output or everypar routines
are not further expanded.

* Things like \count and \dimen and \skip should not appear except in the
context of numbers, dimens, and glue; for example, you shouldn't say "\dimen 5"
or even "\count 5" in the midst of a paragraph. But if you say "\the\count5",
the paragraph will get the (signed decimal) value of the counter; if you say
"\the\dimen10" you will get text like "3.14159pt"; and "\the\skip12" yields text
like "-5.00000pt plus 3.40001fil". In \xdef and \write, TeX82 will expand
occurrences of \the, using the current values, but \count and \dimen and
\skip will not be expanded. This gives you a little more control over what
gets expanded.

* \thebox is changed to \lastbox (avoids confusion with \the).

* \minusthe is like \the, but gives the negative value.
* An array of 256 dimension values is introduced, called \dimen0 to \dimen255.
Furthermore `3.5dm8' is a dimension equal to 3.5 times \dimen8. These join
\count0 to \count255 and \skip0 to \skip255, so we now have 256 of each basic
quantity (instead of 10, as in TeX80). This applies to \count, \dimen, \skip,
\write, and \box. PLAIN.TEX contains macros for allocating a new \count or
\dimen, etc.

* mu-glue and ordinary glue are unmixable, since the rules are so much cleaner
and clearer this way: There are 256 \muskip registers, which take glue whose
units are mu (instead of pt, etc.). It is now illegal to do things like
\hskip\the\thinmskip or \mskip\the\baselineskip.

* Operations on \count, \dimen, \skip, and \muskip are extended, so that we now
have a complete set:

\setcount <digit> [=] <number>
\advcount <digit> by <number>
\multcount <digit> by <number>
\divcount <digit> by <number>

\setdimen <digit> [=] <dimen>
\advdimen <digit> by <dimen>
\multdimen <digit> by <number>
\divdimen <digit> by <number>

\setskip <digit> [=] <glue>
\advskip <digit> by <glue>
\multskip <digit> by <number>
\divskip <digit> by <number>

\setmuskip <digit> [=] <mathglue>
\advmuskip <digit> by <mathglue>
\multmuskip <digit> by <number>
\divmuskip <digit> by <number>

Note that \specskip has changed its name to \setskip. The division operations
truncate towards zero. The = sign in \setcount, \setdimen, \setskip, \catcode,
\font, \openout, \let, and in other similar things, is now optional.
Furthermore an equals sign is optionally allowed now after \tolerance,
\hsize, \baselineskip, \setbox (the new name for \save), etc.
* If you put \global in front of \def or \let or \catcode or \tolerance or
\baselineskip or even \parshape or \hangindent or a font identifier, the
definition will now be global. Otherwise the definition is local (except
for \gdef and \xdef). This is a change in the case of dimension
parameters: \varunit, \parindent, \lineskiplimit, \mathsurround,
\maxdepth, \topbaseline, \hsize, \vsize; you should put \global in front
of these to ge␈ the former behavior. You probably wanted the former
behavior only when changing \hsize or \vsize in an \output routine.

* \edef is a local \xdef. Both \edef and \xdef can now take arguments like \def
and \gdef.

* Two other modifiers can be placed in front of \def, \gdef, \edef and \xdef:
\long means that the arguments to the macro are allowed to contain
\par tokens; formerly this was always allowed, but now it
is permitted only for "long" macros. Otherwise TeX will now
stop when it sees \par going into an argument, presuming
that a right brace was forgotten. This detects one of the
most frequent errors made by TeX users, before it propagates
to overflow the memory.
\outer means that the macro being defined is not allowed to appear
subsequently either in an argument or in the right-hand side
of a definition or write text, or in the preamble of an
alignment. In other words, the macro should appear only at
"quiet" times. This is another way to catch missing braces
before too much damage is done. It used to be applied at the
end of every page, but most TeX users don't use a page-oriented
editor like E; therefore TeX82 does not treat file pages as
an integral part of its control structure.

* \begingroup and \endgroup provide an alternate way to enter and leave groups
for locally defined values. A \begingroup will not match a }, nor will { match
\endgroup; the former gives the message "Missing \endgroup inserted" when the }
occurs, and the latter inserts a "missing" }. Note that you can introduce
\begingroup in one macro and \endgroup in another.
* If you say \message{text}, the terminal will display " text" immediately. For
example, the new version of PLAIN.TEX contains message statements so that
when INITEX inputs the file your screen looks something like this:
(PLAIN.TEX preloading macros, fonts, codes, hyphenation)
instead of "(plain.tex 1 2 3 4 5 6)". (Note that you don't say \input basic any
more, and PLAIN.TEX is already preloaded when you run TeX, as explained below.)

Here's an example macro that displays names of sections when you get to them in
a paper you are TeXing:
\outer\def\section#1{\vfill\eject\message{#1}\centerline{\bf#1}}

* You can also say \errmessage{text}, which causes a TeX error message like
! text.

* \catcode<n> replaces \chcode <n>. \mathcode<n> replaces \chcode <n+128>.

* A new catcode value 14 denotes a character that is better for comments than
the old code 5. A character of code 14 denotes end of the current line (i.e.,
ignore the remainder of that line), without inserting a blank space, and without
considering that line to be all blank. Thus, if % is assigned type 14, you can
have lines that are completely comments by starting them with %, without having
this line come out as \par; and you can also end a line with % without having a
blank space inserted there.

* Another new catcode value, 15, denotes an invalid character. When such a
character is input, TeX82 issues an error message.

* Here's something that was NOT put into TeX82: It wouldn't be hard to make TeX
understand \escape to mean 0, \opengroup to mean 1, ..., \active to mean 13,
\comment to mean 14, and \invalid to mean 15; then you could say, e.g.,
\catcode'14=\active in the example above. But it seems wrong to make
\catcode too easy, since that will only encourage more people to fiddle
with the \catcode table. Let's leave this a black art, to be resorted to
only with reluctance in times of emergency.

* You CAN do certain things now in horizontal mode, e.g., \vfill; TeX82 will
silently insert the \par you forgot.

* \discretionary{#1}{#2}{#3} makes discretionary characters other than hyphens.
It means the text should either contain #3 without a break, or else it should
contain #1, then a break, and then #2. For example, \- is equivalent to
\discretionary{-}{}{}. The parameters #1, #2, and #3 may not contain anything
but letters and otherchars (not spaces or penalties, etc.); they need not all be
in the same font, and TeX will insert ligatures and kerns within them if
necessary. For example, the correct way to specify the hyphenation of
"difficult" is "di\discretionary{f-}{fi}{ffi}\-cult". In German, the correct way
to specify hyphenation of "backen" is "ba\discretionary{k-}{k}{ck}en";
presumably if we were doing a lot of these we would define a \ck macro so that
one could type "ba\ck en" or "ba{\ck}en". The third part of a \discretionary
must be empty, in math mode.
* The internal character set used by TeX82 is the same regardless of the
external character set. There is no longer a difference like "\catcode'176" for
right brace that applies only at SAIL! Right braces and underlines and tildes
and notequals and a few others have been a source of problems that have now gone
away. Furthermore there is now a way to input an ascii control character to any
version of TeX82 by typing, e.g., ↑↑A.

TeX82 assumes that all of the standard ascii characters, shown in positions 040
through 176 below, are available; these characters are always converted to their
standard ascii codes. For example, a TeX user who types A is asking for
character 65 of the current font, even though the A might have entered the
computer in EBCDIC or some other code. Non-standard-ascii characters might also
be readable on some implementations of TeX. In such cases they should have the
significance stated below, for best results; and all characters that cannot be
converted to a compatible TeX code should be converted to 177.
ascii TeX description catcode mathcode (plain TeX)
000 ↑↑@ null ignore bin401
001 ↑↑A downarrow submark rel443
002 ↑↑B alpha other ord213
003 ↑↑C beta other ord214
004 ↑↑D and other bin536
005 ↑↑E not other ord472
006 ↑↑F epsilon other ord217
007 ↑↑G pi other ord231
010 ↑↑H backspace,lambda ignore ord225
011 ↑↑I tab,gamma space ord215
012 ↑↑J linefeed,delta ignore ord216
013 ↑↑K uparrow supmark rel442
014 ↑↑L formfeed,+/- endline bin406
015 ↑↑M carriage-return endline bin410
016 ↑↑N infinity other ord461
017 ↑↑O partial other ord245
020 ↑↑P subset other rel432
021 ↑↑Q superset other rel433
022 ↑↑R intersection other bin534
023 ↑↑S union other bin533
024 ↑↑T for-all other ord470
025 ↑↑U there-exists other ord471
026 ↑↑V circle-times other bin412
027 ↑↑W leftrightarrow other rel444
030 ↑↑X leftarrow other rel440
031 ↑↑Y rightarrow other rel441
032 ↑↑Z notequal other rel434
033 ↑↑[ escape,diamond action bin567
034 ↑↑\ less-or-equal other rel424
035 ↑↑] greater-or-equal other rel425
036 ↑↑↑ equivalence other rel421
037 ↑↑_ or other bin537
040 space space ord464
041 ! exclamation other close041
042 " double-quote other active ↑{\prime\prime}
043 # hashmark param ord561
044 $ dollar-sign math ord577
045 % percent-sign comment ord045
046 & ampersand align ord046
047 ' apostrophe other active ↑\prime
050 ( left-parenthesis other open050
051 ) right-parenthesis other close051
052 * asterisk other ord052
053 + plus-sign other bin053
054 , comma other punct054
055 - hyphen,minus-sign other bin400
056 . period other ord056
057 / slash other ord057
060 0 zero other var060
. . .
071 9 nine other var071
072 : colon other rel072
073 ; semicolon other punct073
074 < less-than-sign other rel074
075 = equal-sign other rel075
076 > greater-than-sign other rel076
077 ? question-mark other close077
100 @ at-sign other ord574
101 A uppercase-A letter var301
. . .
132 Z uppercase-Z letter var332
133 [ left-bracket other open133
134 \ backslash control bin404
135 ] right-bracket other close135
136 ↑ caret supmark ord017
137 _ underline submark ord465
140 ` reverse-apostrophe other open140
141 a lowercase-a letter var341
. . .
172 z lowercase-z letter var372
173 { left-brace open open546
174 | vertical-line other ord552
175 } right-brace close close547
176 ~ tilde other rel430
177 ↑↑? invalid invalid ord573

As before, the mathcodes (which replace Appendix F8 of the old TeX manual)
are relevant only when the catcode is letter or other. (See below for the
new catcode values.) Two possibilities are given for codes 010, 011, 012,
014, 033, 055; at most one of these should be chosen, and if both are
present on some system keyboards the other should probably be disallowed
for TeX input (mapped into 177). However, since a user can change any
catcode and any math code, strict conformity with these interpretations
isn't absolutely necessary. To convert a file into a format that all TeXes
can read, one should change null into ↑↑@, downarrow into ↑↑A, and so on.
If a character set contains uparrow but not caret (e.g., the SAIL system
falls into this category), the uparrow should be considered an ascii
caret; code 013 will be used only if both uparrow and caret are present,
as they are at MIT. Incidentally, this internal coding scheme is based on
a scheme used at MIT, since the MIT code is faithful to ascii while
allowing additional visible characters that are extremely convenient.

An appearance of ↑↑A is equivalent to an appearance of ascii code 001, if the
current catcode of ↑ is supmark. In particular, if somebody in a foreign country
with more than 26 letters in the local alphabet wants to make \catcode ↑↑A =
letter, then control sequences like \a↑↑A↑↑Ab (a four letter word) are
permissible.

TeX82 puts 015 (ascii carriage-return) at the end of each line, except for the
lines that are inserted with "i" after error messages. If the final character of
the line is currently catcoded to be an escape character (e.g., if you end an
error-insertion with \, or if you do \catcode'15=0), the result is equivalent
to \csname\endcsname (control sequence of length zero).

Of course, users are expected to type \ne instead of ↑↑Z if their system's
character set doesn't contain a not-equal sign; TeX82 recognizes ↑↑Z as ascii
032 primarily to make it possible for straightforward translation of TeX files
from one system so that they will work on another.

Some files contain ascii 014 (form-feed) characters as page marks. Such
characters are ordinarily treated like carriage-returns, since the initial
catcode for 014 is carret. In order to get TeX82 to do the error checking at the
end of a page, as the old TeX did, you can say
\catcode'14=13 \outer\def↑↑L{\par}
* Note that the backslash character is now predefined as an escape character
when TeX82 begins. The old idea about letting the user's first nonblank
character be the escape has been abandoned. Furthermore TEXPRE has been replaced
by a version of TeX called INITEX that allows an entire macro package to be
preloaded; this macro package can define its own catcodes and mathcodes. The
normal version of TeX already has "PLAIN.TEX" preloaded; the normal version of
AmSTeX already has the AmSTeX macros and fonts preloaded.

The new rule for starting TeX is this: When you're running INITEX or a version
of TeX that has a preloaded format, you can request format file f.fmt by typing
`&f' after the ** prompt. When you're running VIRTEX, format file plain.fmt will
be loaded unless you type `&somenonplainformatname' after the ** prompt. Thus,
for example, the following ways of starting TeX are equivalent at SAIL and
similar sites:
tex paper
r tex;paper
r tex
** paper
r tex
**\input paper
(The asterisks here are TeX's prompt character. On TOPS-20 what used to be
"@tex paper/", indicating batch mode, is now "@tex \batchmode\input paper".)

* There are four new primitives \batchmode, \nonstopmode, \scrollmode,
\errorstopmode that represent increasing amounts of interaction. \batchmode and
\nonstopmode will never stop for any reason; \batchmode omits printing anything
on the terminal (but the .log file gets everything, as usual). These nonstop
options are intended for overnight batch processing. \scrollmode doesn't stop
for error messages, but it does stop if files can't be found, or if \pausing is
nonzero. \errorstopmode is the default. If you aren't in \errorstopmode, your
.log file will contain "help" messages for all of your errors. These modes are
global (they don't revert at end of group). They can be set in a format file;
thus you can have a format that implies batch processing. But you could override
that, e.g. by running "batchtex \errorstopmode \input paper". (This example
assumes that batchtex is a program representing "virtex&batch", i.e., a virgin
TeX with batch.fmt loaded and then the core image saved.)

* \dump will save TeX's current memory contents. \dump is essentially like \end
(it's the last thing you do with INITEX), and you don't specify a file name. If
your input was named foo, your output file will be named foo.fmt. Such files are
now called format files. This is allowed in INITEX only, and only at very quiet
times (i.e., at group level 0 in vertical mode with nothing on the current page,
etc.). The file name will be printed later when these memory contents are
loaded in a production version of TeX; for example, if you say "\dump" on
March 1, 1982, the TeX that uses the dumped file might begin with the line
This is TeX, Version 1 (format=plain 82.3.1)

* Actually the program name TeX now stands only for versions of TeX82 that have
PLAIN.TEX preloaded. Other preloaded versions (e.g. AMSTEX) will usually exist
too. If your operating system does not allow a program to start with its memory
preloaded, you will have to call a "virgin TeX" program VIRTEX that first wants
to see the name of a format-dump file (e.g., PLAIN or AMSTEX). In this case a
typical calling sequence might be "@virtex &amstex paper". If no format
is given, "&plain" is assumed. If your operating system is nice enough to
allow preloaded programs, a typical way to create the program TeX would be
to say "@virtex &plain" followed by something like "control-C" and "save tex".

* \indent takes you from vertical mode to horizontal mode and indents the
paragraph; this can be used if the first item in the paragraph is in an
\hbox or \vbox. You can also use \indent in horizontal mode to stand for
"\hbox to\the\parindent{}".

* If you end the parameter part of a definition with an additional #, the
argument-matching process will terminate on the next left brace. For example,
in
\def\chop to #1#{\chopp{#1}}
the call "\chop to 2in{x}" will expand to "\chopp{2in}{x}". The definition
\def\mac#{why}
will subsequently issue an error message if "\mac" is not followed by "{".

* The "texinfo" that is given with each font (see Appendix F of the METAFONT
manual) can now be changed by a TeX user program; there was no way to do this
before except by making a new TFM file. Say
\texinfo <parameternumber>=<dimen>
For example, \texinfo\ff3=4pt sets the stretch component of spacing to 4pt in
font \ff. (Parameter 1, the "slant", is unitless but you should give its
value in units of points.) You can use this feature to adjust math-mode
positioning of subscripts, etc., by changing the parameters in mathsy and
mathex fonts. Note that texinfo is global, it does not get reset at the
end of a group. You can also say \the\texinfo<parameternumber>.

* New dimension parameters \hfuzz and \vfuzz specify the tolerance for printing
a diagnostic message about overfull boxes. If the box is overfull by this amount
or less, no message is printed. Default is ".1pt", which was the old TeX
standard. If you say \hfuzz 8000pt, you probably won't see any overfull boxes.
If you say \hfuzz 0pt, you will see all of them, including a few that you didn't
know about last year.

* Another new dimension parameter \overfullrule specifies the width of a rule
that is added at the right end of overfull hboxes. This rule has the height and
depth of the box. If \overfullrule is zero or negative, or if the amount of
overfullness does not exceed \hfuzz, no rule will appear. Default is 5pt, which
gives a big black mark to help you spot overfull boxes.

* The "overfull box" warning messages will be given in a new form that simply
gives the characters in the box; for example,
Overfull \hbox, 3.3326 points too wide (in paragraph of lines 210--216):
\ff This is the text of a line that was over-full for some rea-son.
Discretionary hyphens are shown as real hyphens, so that you can see what
hyphenation TeX was trying. The error-transcript file gets both this message
and an old-style description of the overfull box in detailed diagnostic dump
format.

* Another new control sequence, \relax, does nothing at all. Thus, if you want
to disable the action of a control sequence, you can \let it be \relax.
* Up to 256 fonts may be used, and each font may contain up to 256 characters.
(Characters numbered 128 to 255 can be accessed either via ligatures or
charlists or with the \char command.)

* \leftskip and \rightskip specify glue to be placed at the left and right of
each line of a paragraph. This provides better ways to do ragged right setting,
and it makes changes to \hsize less necessary.

* \lastskip gives the value of the previous item in the current horizontal or
vertical list, if that item was glue, otherwise it yields the value 0pt. Thus,
to get the effect of \unskip in vertical mode, say
"\penalty100000\vskip\minusthe\lastskip".
You can also do things like "\ifdim\the\lastskip > 5pt ... \else ... \fi",
so that one macro can make decisions about spacing based on what has gone
before. (This finally solves the long-standing problem about spacing
after theorems that end with a displayed equation.) Note that
\the\lastskip is permitted, except in \write statements.

* \sqrt signs in TeX82 are positioned differently in their boxes: The baseline
now comes exactly at the bottom of the place where the vinculum (i.e., the rule
over the operand of \sqrt) is to be joined. This means that no rounding errors
will be possible and perfect alignment will be obtained at all resolutions.
Pre-82 versions of TeX will still work (subject to rounding) if the height of
the box is the thickness of the rule.

* The error transcript files are no longer called "errors.tmp". Your output file
and transcript file will be "paper.dvi" and "paper.log" if your first line of
TeX input specifies \input paper. The default name "texput" is used whenever no
other appropriate name has occurred before TeX reads line two of its input. (TeX
can't wait any longer, since line one has to be put into the transcript file,
and the transcript file has to have a name before it gets information.)

* \hyphenation{word list} can be used to override TeX's hyphenation algorithm;
for example, to specify hyphenation of the words "hyphenation" and
"exceptions" one can write
\hyphenation{hy-phen-a-tion ex-cep-tions}
A new hyphenation algorithm devised by Frank Liang is used in TeX82; this one
extends much more readily to other languages. Words containing ligatures can now
be hyphenated automatically, even difficult words like "difficult".

* If two fonts are specified with the same name and point size, only one will be
loaded.

* \sfcode <char number> is the spacefactor code for that character, times 1000.
For example, the spacefactor code for period and question mark is normally 3000,
for comma 1250, for right parenthesis 0 (meaning do not change the space
factor), and for most characters it is 1000. In TeX82, you also say
"\spacefactor 1234" instead of "\spacefactor 1.234".
* TeX82 has a new "help" facility available on error messages. If you type "h"
after an error, you will (usually) get further explanation of what the error
means, together with suggestions about how to proceed.

* \↑↑\ and \↑↑] have gone away, to the delight of people who don't have nice
ways to type ascii control characters. Instead, \nonscript in math mode precedes
a space of any other type, making that space zero in subscript styles. Thus, the
conditional thin space is now "\nonscript\mskip\the\thinmuskip", and conditional
negative thin space is "\nonscript\mskip\minusthe\thinmuskip".

* \thinmuskip, \medmuskip, \thickmuskip are now definable like other glue
parameters such as \baselineskip. The units should be in mu. For example, one of
the defaults is \thickmuskip 5mu plus 5mu. The old "mathspace" parameter in
symbol fonts (see METAFONT manual p99) is no longer used.

* There's a new way to get up to 4096 more math symbols in all three sizes, by
defining font families 0 to 15. For example, suppose that fonts \fA, \fD, and
\fF are Fraktur alphabets in 10pt, 7pt, and 5pt sizes. Then you can say
\textfont 5=\fA \scriptfont 5=\fD \scriptscriptfont 5=\fF
which is something like the code "\mathrm adf" in the old basic.tex. Now if you
say "\fam5" in math mode, you get characters from font \fA, \fD, or \fF,
depending on the size. For example, "{\fam5 B_b}" would give Fraktur B in
10pt with a subscript Fraktur b in 7pt. The rule is that a family
specification overrides the normal family for symbols of class var (see below).

Note that we can now say \def\rm{\fam0\ff} together with
\textfont 0=\ff \scriptfont 0=\fg \scriptscriptfont 0=\fh
and then it's possible to say, e.g., "\def\max{\mathop{\rm max}}" instead
of resorting to "{\char`m \char`a \char`x}" in order to achieve size-switching.
This extension also makes ligatures and kerning available in math mode.

* The use of certain families is predefined. Family 2 specifies the `mathsy'
fonts used for symbols; family 3 specifies the `mathex' fonts used for large
delimiters; and the other 14 families can be used in any desired fashion.
Instead of `\mathsy uxz' one now says
\textfont 2=\fu \scriptfont 2=\fx \scriptscriptfont 2=\fz
and these assignments are local (they go away at the end of a containing group).
You must have \textfont2, \scriptfont2, \scriptscriptfont2, \textfont3,
\scriptfont3, and \scriptscriptfont3 defined before using math mode, since the
parameters of these fonts contain the values TeX needs for math spacing.
(\scriptfont3 and \scriptscriptfont3 are now supposed to be math extension
fonts, as well as \textfont3, because TeX82 will use smaller extension-type
features in script and scriptscript styles; for example, the default rule
thickness in a subscript is a parameter to \scriptfont3, while TeX80 had only
one extension font for all three sizes.)
* The \mathcode now has a 15-bit number as its value. The first three bits
specify ord, op, bin, rel, open, close, punct, and var (where var is like ord
but it subsitutes the "current" family for the stated one). The other twelve
bits specify a "math character", with four bits for the family and eight for the
character. For example, character '100 in family 5 is '2500; of course, this
reads a little better in hexadecimal: character "40 in family 5 is "540.

You can say \mathchar followed by such a 15-bit code, to get the equivalent of
typing a character with that math code. Thus, one can now say, e.g.,
\def\cdot{\mathchar '10001 }
instead of \def\cdot{\mathop{\char1}} as formerly. The control sequence \char
is no longer allowed to take values greater than 255.

* If the first three bits of a mathcode are 7 (case "var"), it's the same as 0
except that the current value of integer parameter \fam replaces the
family specification (if it is between 0 and 15). The current family is
set to -1 whenever math mode is entered.

* A \mathcode can also be '100000; this causes the corresponding character to
behave as if it had catcode 13 (if it is a letter or otherchar).
For example, plain TeX defines the mathcode of ' to be '100000,
and control sequence ' is defined to be ↑\prime,
so that you can say f'(x) instead of f↑\prime(x). In this way
you can have characters that are active only in math mode. (Furthermore,
an octal constant like '77777 will still be acceptable in math mode.)

* New primitive \mathchardef will save lots of memory if you have lots of
control sequences for math things like \alpha, etc.:
\mathchardef\cs[=]<fifteen bit number>
will have the effect of
\def\cs{\mathchar <fifteen bit number> }
and it will use none of TeX's memsize. (By contrast,
\def\cs{\mathchar "1234 }
takes up 9 words of memory!)

* The \radsign parameter goes away. Instead, one says \radical followed by a
delimiter code. Delimiter codes may be used also after the control sequence
`\delimiter' in connection with \left and \right and \atopwithdelims.

A delimiter code is a somewhat esoteric 24-bit number. The first twelve bits
specify a `small' character, and the last twelve bits specify a `large' one.
When TeX chooses a delimiter, it searches in the following way until finding the
first one large enough: First it looks at the `small' character in the current
size of the family, then (if the current size isn't text size) it looks at the
small character in the next larger size, and so on until coming to text size. If
a suitable delimiter has still not been found, the same search is carried out
starting at the `large' character. If any of the characters looked at is part of
a "charlist", the list is searched before moving on. If the small or large
character is zero, it is ignored; thus, you can't use character 0 in family 0 as
a delimiter.

For example, \sqrt is equivalent to \radical '11601560 in the Computer Modern
fonts; the 1160 specifies \fam2\char'160, and the 1560 specifies \fam3\char'160.
Since '160="70, we can also write this as \radical "270370.

If \delimiter <delimiter code> appears in a formula in some place not
controlled by \left or \right or \atopwithdelims, it is actually a 27-bit
code. The least significant 12 bits are ignored, and the leading 15 bits
are used like a \mathchar (thus, they specify a category as well as a
family and character). The reason is that one can now say, e.g.,
\def\lfloor{\delimiter '411421404 }
so that one can say both \left\lfloor and simply \lfloor.

A \delcode is also given for letters; \left and \right and \atopwithdelims will
use this code as a delimiter code if they are followed by an "otherchar". For
example, the Computer Modern fonts use \delcode`(='501400, assuming that family
0 contains the ordinary roman alphabets. Initially, \delcode is negative for
all characters; this denotes an invalid delimiter.

All this bit hackery is, of course, unfriendly looking, but the goal is to make
it possible for macro packages to define the friendly codes without taking up
much memory space inside of TeX. All of the TeX control sequences that used to
be predefined for Computer Modern are now unbundled so that arbitrary encodings
can be used. One of the embarrassing limitations of TeX80 was that it could
handle delimiters only between '142 and '153 in the symbols font, and it
insisted that these `small' delimiters had corresponding `large' ones in
positions '004--'015 of the mathex font!
* The spacing in math formulas is unbundled too; there are three parameters
\thinmuskip, \medmuskip, \thickmuskip to specify the spacing in formulas like
$x\log x$, $x+x$, and $x=x$, respectively. One defines these using "mu" units,
e.g. `\medmuskip = 4mu plus 2mu'. The control sequences `\,' `\>' and `\;' in
math mode yield spaces of these three varieties. (These are now defined in
PLAIN, not TeX primitives. The previous meaning of \> is now rendered
`\nonscript\>'.)

The following example shows a feature that is NOT allowed:
$a \save1\hbox to 18mu{}
\ifdim 1wd1=10pt{\gdef\x{\over b} \else{\def\x{}}
\x$
Whoever wrote this was trying to be clever and discover whether TeX was
in \textstyle. But the program is self-contradictory, because if the "a" is in
10pt text style the formula changes itself to $a\over b$ where the "a" is in
script style, so the formula changes itself to $a$ where the "a" is in text
style, so... Constructions like this show why TeX does not allow variable
dimensions like mu except in very restricted ways like \mskip.

* New dimension parameters
\scriptspace {this amount is placed at the right of all subscripts
and superscripts; TeX80 used about .45pt always}
\nulldelimiterspace {this amount is used before and after all fractions
defined by \atop, \over, \above, and for all
"." delimiters}
\delimitershortfall {see the next parameter}

* New integer parameter \delimiterfactor. When TeX computes the size of
\left and \right delimiters, it computes delta1=twice the maximum distance
of the enclosed formula from the "axis". (The axis is where the fraction
line would go.) Let delta2=delta1*(delimiterfactor/1000) and
delta3=delta1-delimitershortfall. The delimiters will be as small as
possible provided that their height+depth exceeds both delta2 and delta3.
(TeX80 took delimiterfactor=900 and delimitershortfall=1ex; TeX82 lets the
user twiddle with these magic numbers.)

* When unscripted letters occur in math mode, they now are adjusted for
ligatures and kerns. This means, for example, that "df" will be spaced better,
once appropriate kerning information has been added to the math italic fonts.
The (new) rules for spacing are this: If there's no kerning specified, add the
italic correction to every symbol in math mode; otherwise use the kern (without
the italic correction). However, a subscript is moved left by the amount of
italic correction. In formulas like $P_2↑2$, the 2's will no longer be directly
above each other (the sub-2 will be to the left of the sup-2 by the amount of
the italic correction). TeX80 put them above each other, thereby following a
long-standing convention (cf. Oxford book by Chaundy et al.), but this usually
turned out to be undesirable, so people started to write $P↑2_{\!2}$ all the
time. If anybody really wants it the old way, they can get it by $P{}↑2_2$.

* "\comb" is changed to "\atopwithdelims". There's also "\overwithdelims" and
"\abovewithdelims".
For example, {1\overwithdelims[]2} is sort of like \left[1\over2\right].
And "\abovewithdelims.." is equivalent to "\above".
* The maximum penalty has been raised from 1000 to 10000; thus, "\penalty 1000"
no longer absolutely prohibits a break, but "\penalty 10000" does. This number
10000 is being used elsewhere in TeX82 also: For example, \tolerance or
\pretolerance of 10000 is equivalent to saying "use all possible line breaks,
regardless of how much stretching is necessary." A \penalty of -10000 (or less)
is equivalent to the old \eject in vertical mode, and to the old \linebreak in
horizontal mode. These are no longer primitives of TeX82.

* The \pagebreak feature is also eliminated, in favor of a much more general
feature. You can say \vadjust{vertical list} in the midst of any paragraph, and
whatever is in the specified vertical list will be placed immediately following
the box for its line when the paragraph has been made. For example, \pagebreak
is now written \vadjust{\penalty-10000}. You can use this feature to do things
like insert extra space between lines of a paragraph, something like \noalign
does for alignments.

* \ifvoid n tests if \box n is not present. All boxes are initially void;
a null box (made e.g. by \hbox{}) is empty but nonvoid. A box becomes void
after it is used.

* \topbaseline has changed to \topskip; thus glue is allowed at the top of a
page. This makes it easy to "bottom justify", for example. (The old \topskip
and \botskip are no longer used.)

* \vtop is now allowed in any mode, just like \vbox.

* \special{keyword arg} is a general extension feature. The keywords are system
dependent, but TeX copies "keyword arg" into the DVI file so that any device
driver that knows your keywords will do the right thing with them. Users should
get together if they want to standardize on various keywords. Examples:
\special{halftone fig22} could mean "insert a halftone from file fig22, with its
reference point at the current reference point"; \special{leftend 2} and a later
appearance of \special{rightend 2} could mean "draw a straight line from
the left reference point to the right one" (the "2" is an identifier to
distinguish this line from another one); \special{message Foo} could mean
"display `Foo' on the console of the printing device"; and so on.
Semantically, \special acts like a box of height, width, and depth zero,
as far as TeX is concerned; the argument in braces is sent to the DVI file
where it is associated with the current reference point.
The length of "keyword arg" must be at most 255 characters.

* \ragged is no longer implemented, since \rightskip does ragged right setting
so much better.

* \ifcase <number> case0 \or case1 \or case2 \else remaining cases \fi
illustrates a new way to choose between more than two alternatives without
too many nested brackets. Any number of cases can be given (but TeX has
to scan by them each time, so it's best to define them as control sequences
if they are long). The \else is optional.
* Major changes have been made to the \output conventions, so that it will be
easier to produce balanced columns and various other things. The old ideas of
\topinsert, \botinsert, \topsep, \botsep are eliminated!

In their place one says \insert n, where n is a box number, e.g. "\insert 250".
Different numbers correspond to different classes of insertions; for example,
one might want to have figures as well as footnotes inserted at the bottom of
pages, and TeX80 used to use the same treatment for both. Under the new
conventions, all \insert 250's that go on a page will appear in \box 250 when
the \output routine starts.

The old idea of "\page" is gone too, and the meaning of \output has changed, so
read carefully: The contents of an accumulated page, exclusive of inserts, is
placed into \box 255, so that the output routine can place this material
together in whatever way it wants. (\insert 255 is not legal.) A new TeX
primitive called \shipout, followed by a box specification (e.g.,
"\shipout\vbox{\box255\box250}") is what actually produces output. Note that
\ifvoid 250 can be used to test whether any \insert 250's have been gathered
for a page. The \shipout command can be used anywhere, not just in \output.
Incidentally, \shipout prevents the old anomalies about the values of \counts
being different from what people thought they would be when \writing
table-of-contents or index data to a file. The default \output routine in TeX82,
if none is specified, is "\output{\shipout\box255}".

Another new primitive, \vsplit, is handy for multi-column input. The command
"\setbox 2=\vsplit 250 to 100pt" will, for example, make \box2 a box whose
height is 100pt, by extracting 100pt worth of material out of \box250. The depth
of \box2 will be at most \splitmaxdepth (using the rules that TeX used for
\page); \vsplit extracts the optimum initial segment of \box250, in the sense
that badness+penalty is minimized and the segment is as long as possible subject
to this condition. After \vsplit has acted, \box250 will contain the residual,
as if a page break had occurred; thus, glue and penalties will be eliminated
following the break, and the first box or rule (if any) will then be preceded by
sufficient glue to position its top baseline (based on \splittopskip).

The main idea of \vsplit is to make it easier for an \output routine to produce
multiple column format. For example, if \box255 is 300pt high, one can get
triple columns by
\setbox 1=\vsplit 255 to 100pt
\setbox 2=\vsplit 255 to 100pt
\setbox 3=\vsplit 255 to 100pt
(Actually it is safer to use slightly less than 100pt here, but the exact
measurements depend on the top baseline and other things.) The remaining
material in box 255, if any, can be put back onto the following page, as we will
see momentarily; and boxes 1,2,3 can be positioned as desired before they are
shipped out.

The \vsplit routine also looks at \marks in the contained box. It sets
\splitfirstmark and \splitbotmark to the topmost and bottommost contained marks;
otherwise it sets these to null strings.

Of course you can't apply \vsplit to a box that was constructed as an \hbox.
There is no \hsplit.

The old \output routine defined a sequence of items in restricted vertical mode,
with the meaning that this sequence would be vboxed and shipped out. The new
\output routine defines a sequence of items in restricted vertical mode, with
the meaning that this sequence will be placed in front of whatever TeX has
accumulated for the following page, including whatever caused a break on the
current page. Thus, if you write "\output{\unvbox255}" you are in serious
danger of getting in an infinite loop.

Consider, for example, what happens if a page break occurs at some glue. If the
output routine leaves some of the tail end of the material from \box 255 in its
vertical list, this material will fit perfectly before the glue that caused the
previous break, since the glue following a break is not eliminated when \box255
was made; glue is simply discarded when it appears at the top of the new page
that is started after \output finishes.

Consider also what happens if a page break occurs at "\penalty-10000". If you
understand what has just been said, you will see that this would cause repeated
looping if the \output routine places something back for the next page, and this
might be a problem. Therefore TeX will change the penalty to +10000 at a break,
and it also sets \outputpenalty to the value of the penalty that actually caused
the break. (\outputpenalty is set to 10000 if it was glue that caused the
break.) Thus, you can restore the effect of a penalty by putting
"\penalty\outputpenalty" at the end of your output routine.

But how does TeX figure out what to put in box 255 and how much of the
insertions to put into other boxes, before it calls on your output routine? The
rules are slightly complicated, but they have been devised to handle a wide
variety of situations, including situations where some inserts span several
columns. It should be possible to do things like put footnotes in two columns
beneath single-column text, or to put single-column footnotes beneath
double-column text, etc. Here's how: We associate \skip n, \dimen n, and \count
n and \box n with \insert n. (a) \count n gives a magnification ratio of
\insert n with respect to ordinary text. For example, if two-column footnotes go
with one-column text, and if footnotes are inserted with \insert250, then
\count250 should be 500. If single-column footnotes or page-wide figures are
being inserted with double-column text, the magnification ratio should be 2000.
(b) \dimen n gives a maximum length of inserts for box n; subsequent inserts
will be carried over to a following page. (c) \skip n gives a correction term
when there is at least one insertion for box n.

The total length of inserts is figured as the sum, over all n such that \insert
n appears on a page, of the \skip n plus (\count n over 1000) times the total
natural height plus depth of all \insert n's, including the original contents of
box n before any inserts were made. The badness of a page is computed from the
amount of the text on the page plus the total length of inserts, and TeX breaks
the page so that badness is minimized. The \vsize is the total of text plus
insertions; if you are using double-column text, your \vsize should be about
twice the actual page height.

If this isn't complicated enough, there is also a rule for splitting inserts, so
that long footnotes can be broken between pages and so that large figures can be
carried over to subsequent pages. Here's the idea: When we are deciding whether
to perform an \insert n or not, we first look to see if previous \insert n's
have all been completed without splitting. If not, this one is carried over to
the next page. If so, this one is put on the current page, if it does not cause
the page to overflow and if it does not cause the maximum (\dimen n) to be
exceeded. In the latter cases, the insertion is \vsplit to the maximum size that
would not cause such overflow. For example, suppose we get to an insertion at
magnification 500 when \vsize minus the current amount of text and the previous
total amount of insertions leaves only 50pt of vertical space left. Suppose the
insertion takes 150pt of vertical space, so that it would take 75pt after
scaling; and suppose that 150pt would not exceed the maximum total size of
insertions for this box. Then we essentially \vsplit the insertion list to 100pt
(this will scale down to 50pt), after which the actual length of the insertion
will be computed as its natural height plus depth (which might be different from
100pt). The remaining part of the insertion will be corrected for top baseline,
etc., as in an ordinary \vsplit, if this broken insertion is actually chosen.
But TeX will use broken insertions only if they lead to the minimum badness for
the resulting page.

If several \insert n's appear on the same page they are concatenated together
with no baselineskip correction between them. So you should use struts to
produce the correct positioning.

Insertions are treated the same whether they appear in horizontal or vertical
mode. A "floating" insertion turns out to be a special case of a broken
insertion, whose first component is a null box.

TeX doesn't put waiting insertions into \box255, it leaves them on the list for
the subsequent page. If insertions appear in a box that is being vsplit, they
are ignored.

It's too bad that these rules came out so complicated, but in simple cases the
output routines will now be quite simple, and the manual will have enough
examples to make things clear (I hope). Nothing simpler than this seems to
provide the other features that people have been demanding, and the total amount
of programming for the TeX82 page builder is not much more than there was in
TeX80.
* In vertical mode, you can say \prevdepth=3pt to make TeX82 act as if the
previous box had a depth of 3pt, when computing the glue between boxes to
achieve the baselineskip. If you set \prevdepth to a value less than or equal
to -1000pt, the baselineskip calculation will not be made. (This is the
case at the beginning of a vbox, or just following an hrule.)

* You can say \the\spacefactor and \the\prevdepth, if you are in horizontal or
vertical mode, respectively.

* New diagnostic features: "\showbox 10" will display the current contents of
\box10 on the terminal. There's also \showthe as in "\showthe\count 5" and
"\showthe\baselineskip". Also "\show\cs" to give a symbolic display of the
current meaning of the control sequence \cs. The present \ddt is deleted, and
\showlists exhibits the current activities the way \ddt used to. If you are
trying to diagnose some mysterious behavior, you can say, for example,
"\showthe\texinfo\ff 40" and you will get an error message like "\font \ff has 7
texinfo parameters" (if it has fewer than 40). Incidentally, if your error
message was "\font\fg has 7 texinfo parameters", you would know that fonts
\ff and \fg are being treated identically. (TeX loads only one copy of a
font that you mention twice. If you want to load two distinct copies, so
that you can diddle their parameters independently, you can try something
like this: "\font \ff=cmr10 at 10pt \font \fg=cmr10 at 10.00002pt".)

* If you use a \skip parameter in the context of a dimension, the natural width
is used. For example, \setdimen 5=\the\baselineskip. If you use a \dimen
parameter in the context of an integer, the conversion is in units of sp (scaled
points, 2↑{-16} of a pt). For example, \setcount 10= 1truept would set \count 10
equal to 65536000 divided by \mag.

* When you use \advskip, infinite glue wipes out finite glue.
For example, "\setskip 2=5pt plus 2pt minus 1fill
\advskip 2 by 3pt plus 1fil minus 1fil"
is equivalent to "\setskip 2=8pt plus 1fil minus 1fill".

* \linepenalty is yet another parameter to control line breaking. TeX82 adds
this to the badness before squaring to get demerits. (Previously, I had
\linepenalty=1 always; by setting it a bit higher, like maybe 7 or 8, you tend
to get paragraphs that are set tighter when a line can be saved. I don't think
it's a good idea to make \linepenalty real large, and it would be foolhardy but
weird to make \linepenalty = -10000; this apparently would minimize the number
of lines but maximize the badness!)

* Macro parameters are now delimited by strings instead of single items.
For example, \def\a#1ab{...} followed by \a acaab will set #1 to "aca";
\def\a b#12#{foo#1} followed by \a bbar2baz2{8} will expand to foobar2baz{8};
the latter followed by \a 2... will give an error message (\a not followed
by b). You get the error message only if there's a string before the
first parameter.
* Characters catcoded 13 are not equivalent to one-letter control
sequences. They act like control sequences (e.g., you can use them after
\def and \let), but & and \& will be distinct.

* \everypar{...} inserts its argument into TeX's scanner at the moment TeX has
changed from vertical to horizontal mode. The paragraph indentation will already
appear in the paragraph, unless of course the transition to horizontal mode was
due to \noindent.

* Spanned and omitted columns in alignments: If an entry in an alignment is
`\omit', the preamble text for the column is omitted in this row. The control
sequence \span can be used in place of a tab mark, and the result is that the
surrounding entries are combined together. You can use \omit only as the first
item of a column. If you use \cr in place of a tab mark, the effect is as if
all further columns in that row are omitted; thus, your preamble can specify
more columns than are actually used.

Example: \tabskip 1em plus 1em
\halign to 20em{\ctr{#}&\rt{#}&\lft{#}\cr
AAA&B&C\cr
DDDDDDD\span\omit&EE\cr
\omit\span FFFFF\span\omit\cr
G&\omit H\span III\cr}
where all the letters are 1em wide, say. Let wij be the maximum width of the
entries that span columns i thru j. The first line "AAA&B&C" implies that
w11≥3, w22≥1, w33≥1. The second line says that w12≥7 and w33≥2, and so on;
we find that w11=3, w12=7, w13=5, w22=1, w23=4, w33=2. Column widths are
now assigned from left to right, as follows:
c1=w11
c2=max(w22,w12-t1-c1)
c3=max(w33,w23-t2-c2,w13-t1-c1-t2-c2)
where ti is the natural width of tabskip between columns i and i+1. In this
case t1=t2=1, so c1=3, c2=3, c3=2. This means the natural width of the lines
will be 1+3+1+3+1+2+1=12 ems, so the glue will be stretching to make up the
additional 8ems; each unit of stretch is doubled. When columns are spanned,
however, TeX justifies the material into a box having the appropriate width for
the tabskip glue that was omitted; for example, an entry that spans columns 1
and 2 will be justified to width c1+t1+sf*s1+c2, where s1 is the stretchability
between columns 1 and 2, and sf=2 in this case since the glue is being doubly
stretched. (If the tabskip glue shirinks, we would of course use shrinkability
instead; in this case spanned columns might actually get smaller than their
natural size.)

* Spaces are ignored after tab marks in alignments.

* If every entry in an alignment that uses column j also spans column j+1,
the tabskip glue between columns j and j+1 is effectively eliminated.
Similarly, if column j never appears (because each row had \cr before
getting to that column), the tabskip glue after it is eliminated.
* \hbox par is eliminated! Instead of "\hbox par 100pt{...}", one now says
"\vbox{\hsize 100pt ...}" and the effect is almost the same. The only difference
is that the paragraph or paragraphs in the \vbox will have both their hanging
indentation and looseness (and perhaps also their baselineskip) specified inside
the vbox; this is, of course, more logical than the old rule.

Restricted vertical mode is no longer restricted; it's called "internal vertical
mode". It differs from vertical mode only in not going through the page builder,
and in allowing \unskip as well as \vskip with infinite shrinkability.

* To have the paragrapher work on inserted text (e.g., in footnotes), one writes
"\insert 250{\hsize 200pt ...}", perhaps using the \interlinepenalty parameter
that adds to the penalty between lines of a paragraph (whether in inserts or
not).

* Three new dimension parameters \displaywidth, \displayindent, and
\predisplaysize are assigned values at the beginning of every displayed formula:
(1) displaywidth is the length of the line that will contain the formula before
it is centered; this is usually equal to \hsize, except when hanging indentation
or \parshape are being employed in a paragraph. (2) \displayindent is the
amount by which that line is indented. (3) \predisplaysize is the amount of
copy on the line preceding the displayed formula; this is what is used to decide
between \dispskip or \dispaskip. If the display immediately follows \noindent
or another display, \predisplaysize will be -(2↑{30}-1) sp (the smallest legal
dimension in TeX). Otherwise, if the position of the last box on the previous
line is affected by glue stretching or shrinking, \predisplaysize is set to
+(2↑{30}-1) sp. Otherwise \predisplaysize is set to the natural width that the
preceding line would have if all glue were removed at its right end, plus the
amount of indentation of that line, plus 2ems.

* \setcount, \setdimen, \setskip, \setbox, \output, and \everypar are local
definitions unless specified global. Likewise the results of \advcount, etc.

* The { } in \output still defines grouping (it would be too dangerous to
leave it out, since \output occurs asynchronously), but the { } in \everypar
does not. Grouping is now independent of \if tests, as explained later.

* All of TeX's primitives now mean the same thing in all modes. The control
sequences that were exceptions to this rule have been dealt with as follows:
\ (control space) now means a text space, even in math mode.
\quad is no longer a primitive (PLAIN defines it as \hskip 1em).
\! is no longer a primitive (PLAIN defines it for math mode only)
and \ignorespaces is a new primitive that gobbles spaces.
\- is always a discretionary hyphen; its previously advertised mathmode
function has been taken over by the new \nonscript primitive.
* \accent is allowed only in horizontal mode; \mathaccent only in math mode.
The latter takes a 15-bit math code, the former an 8-bit character code; it's
like the difference between \char and \mathchar.

The \mathaccent primitive will make use of a charlist of characters to choose
the first accent of a list whose successor is either nonexistent or wider than
the formula being accented. Thus, you can have a list of longer and longer
tildes or hats, etc.

* Spacing in math is slightly different: Fractions and \left...\right
subformulas are given type Inner, so there are eight types instead of seven.
(Previously Inner was treated like Ord.) The spacing matrix entries are set so
that there's at least a thin space between Inner and its neighbors, except
in the pairs Open Inner and Inner Close. There's a new primitive \mathinner
analogous to \mathpunct, etc.

* "\vcenter to 100pt" is now allowed in math mode, if anybody wants it.

* "\ifdimen" is renamed "\ifdim" and there's also "\ifnum" replacing "\ifpos".
Examples: \ifnum\count1>5...\else...\fi; \ifdim\dimen3<1.5wd2...\else...\fi.

* "\ifinner" is true if the current mode is internal-vertical,
restricted-horizontal, or non-display-math. Combining this with \ifvmode,
\ifhmode, \ifmmode makes it possible to determine exactly what mode you are in.

* New primitive \kern allows you to specify unbreakable space (without
stretching or shrinking). Thus, "\kern -1pt" is something like
"\penalty10000\hskip-1pt". There's also "\mkern 3mu" in math mode. You are
allowed to use \kern but not \hskip in \discretionary lists. A \kern in a word
does not upset the hyphenation algorithm. You can use \kern in vertical as well
as horizontal lists. It is legal to break at a kern if it is immediately
followed by glue or leaders, provided that it is not preceded by glue, kern, or
penalty.

* New debugging facility \tracingcommands, if nonzero, gives a symbolic
indication of what commands are being obeyed by TeX's main control routine.

* New integer parameter \maxdeadcycles gives an upper limit on how many
consecutive invocations of \output do not cause at least one \shipout. This is
intended to catch unintended loops. Default is 25.

* If \tracingstats > 0, you get to see how close you came to TeX's current
table capacities, in a list of statistics printed at the end of your run.
If \tracingstats > 1, you also get to see the current memory usage
every time you do a \shipout.

* If \tracingparagraphs > 0, you get a huge amount of inscrutable printout
about what the line-breaking algorithm thinks it is doing.

* If \tracingpages > 0, you get similarly inscrutable printout
about what the page-breaking algorithm thinks it is doing.

* But \tracingstats, \tracingparagraphs, and \tracingpages are ignored
unless TeX has been compiled in a "slow version" that actually maintains
these statistics. (The SAIL version of TeX is currently "slow" in this way.)

* \string replaces the next token by its text, with all characters regarded
as type otherchar (except that a space will be of type spacer; it's possible
but not easy to get a space here). For example, \string\abc results in four
characters \, a, b, c. This expansion occurs just as for \number, i.e., when
\string occurs in \xdefs or in horizontal or math mode. (The most common
use of \string is to follow it with a macro parameter.) Caution:
If characters in a control sequence name are nonstandard in ascii, they
will be converted differently at different installations.
* Here is an extension to the language intended to placate people who
have objected to the fact that \write (and \openout and \closeout) only
cause action at the time of the next \shipout. Some applications
call for immediate output, hence a new feature: \immediate followed by
\openout or \write or \closeout causes the output action to take place
without delay. For example, \immediate\write0{x} is equivalent to
\shipout\vbox{\write0{x}} except that the latter also puts an empty
page into the DVI file.

* New parameter \boxmaxdepth affects \vbox: If the depth of the box
would exceed \boxmaxdepth according to the normal rules, the box contents
are shifted up so that the depth is exactly \boxmaxdepth, before setting
the glue. The same applies to \vtop, before adjusting its depth.
The default setting is \boxmaxdepth='7777777777sp (the maximum dimension).

* You can now use \let with non-control-sequences after the = sign.
For example, \let\zero=0 makes the control sequence \zero behave something
like the digit zero; but if you want to make the constant 100 by saying
"1\zero 0" you still have to \def\zero{0}. Thus, this new extension isn't
a big breakthrough, but it does save a bit of space and time inside TeX.
(Incidentally, after \let\zero=0, \zero will not expand to 0 in xdefs.)

* Popular demand wins again: You can now say \csname <string>\endcsname
to manufacture a control sequence name. For example, \csname foo \endcsname
is essentially identical to a control sequence named "\foo " (note that
the space is part of that name!) and \csname foo\endcsname is like "\foo"
and, after \def\zero{0}\def\test{zero}, it follows that
\csname\csname\test\endcsname\endcsname is like "\0". The conversion
from token list to control sequence occurs as if \csname were a
macro being expanded. If the control sequence hasn't been defined before,
it will behave as if it were "\relax".

* In the preamble to \halign or \valign, the primitive \span would normally
make no sense. But it causes TeX to expand the following token, instead of
just copying it, before inserting that token in the preamble.
(Previously this was possible only with a dirty "\tabskip" trick, since
TeX expands whatever follows "\tabskip 0pt" looking for "plus".)
* Syntactic conditionals!
For years people have been asking for TeX to treat conditionals in its
"mouth" rather than in its "stomach", and I have been fending them
off. But starting with Version 0.8 of TeX82, \if tests are made at the
time of macro expansion rather than as part of the semantic processing in
horizontal or vertical or math mode.

Instead of writing \if...{..a..}\else{..b..} the new syntax is
\if... ..a..\else..b..\fi
(with \else optional if ..b.. is empty).

Whenever TeX is reading material in a mode where macros are now expanded,
it will process conditionals somewhat as though they were macros. Namely,
\if... results in evaluating the condition and skipping code if the condition
isn't true (skipping to the next \else that isn't enclosed by \if..\fi
brackets); \else, \or, and \fi switch in the appropriate way between
reading and not reading text.

Braces need not be properly nested inside the conditionals, nor do \if...\fi's
need to be properly nested in the replacement texts of macros.
(Having these two types of nesting independent of each other has proved
to be important in many existing macro processors. Caveat implementor.)

People who try things like \expandafter\else\if... should be shot.
(Unless it turns out that this is useful?)

* Another token-list parameter \tokens is definable like \everypar.
Its only use is for things like \the\tokens (which, in an \xdef,
emits the current value of \tokens without further macro expansion).

* Another test, \ifcat, is like \if but it tests the catcodes of the
characters, not their ascii codes.
* \everymath{...} inserts its tokens into TeX's scanner just when
non-display math mode has been entered. \everydisplay{...} does likewise, but
for displays. For example, \everymath{\fam0} sets up family 0 instead of
family-1 as the default for letters; you can also use \everymath
to redefine active characters that you want to behave differently
in math mode.

* \futurelet\a followed by tokens b and c has the effect of "\let\a= c"
followed by tokens b and c. You can use this to look ahead at the
next token after a macro; it's for hackers. (I put it in because it
is easy and because it might allow me to solve some problem next year.)

* \prevgraf is an internal state variable something like \prevdepth and
\spacefactor. It represents the number of lines in the most recently
completed paragraph or partial paragraph. You can use it to provide
"memory" between paragraphs with respect to \parshape and \hangindent.
For example, if you say "\indent \prevgraf=15 This new paragraph..."
the new paragraph will be formatted as if 15 lines had already been
completed. You can look at \the\prevgraf to see how big the last
paragraph was.

* \endinput (in any mode) forces end-of-file whenever the next line of
\input has been fully read. This omits the all-blank line that is normally
appended at the end of a file that terminates normally. \input is now
allowed in any mode, not just vertical mode.
* \hangindent <dimen> no longer looks for keywords "for" and "after"; it's
just like all of TeX's dimen parameters. \hangafter is another parameter
that provides the missing information. Suppose \hangafter equals n.
If n is nonnegative, hanging indentation applies to lines n+1, n+2, ...;
otherwise hanging indentation aplies to lines 1, ..., |n|.
For example, the old "\hangindent 10pt for 2" now becomes
"\hangindent=10pt \hangafter=-2". The old "\hangindent 20pt" (without
"for" or "after") is unchanged, because TeX sets \hangafter=1 after
each paragraph and whenever entering internal vertical mode.
[Reason for this change: It became all too obvious that the old system
was a kludge, when I tried to describe it in the new manual.]

* \clubpenalty (a new parameter) applies after the first line
of a paragraph; \widowpenalty before the last line. (These used to
both be called \widowpenalty. Geoffrey Glaister, U. of Calif. Press 1979,
Glossary of the Book, distinguishes in this way between "club lines"
and "widow lines". The TeX manual sets \clubpenalty=10000 to suppress
breaks after the first line of paragraphs that have dangerous bend
on their first two lines.)

* \jobname returns the name of this TeX job. For example, if the output
goes to file foo.dvi, \jobname is "foo".

* \pagetotal and \pagegoal are internal dimensions representing the
natural height and goal height of the current page. Previously you
could see these only as a result of \showlists; now you can even
set them. If a page is empty, \the\pagetotal is 0pt and \the\pagegoal
is -1000pt. Setting \pagegoal has no effect on an empty page, since
\pagegoal gets set to \vsize as soon as the page gains its first item.
Besides \pagetotal, there is also \pagestretch, \pagefilstretch,
\pagefillstretch, \pagefilllstretch, and \pageshrink, denoting the total
flexibility of glue on the current page; and \pagedepth is the depth.

* \deadcycles is an internal integer that is set to zero on each \shipout
and increased by 1 each time your \output routine is invoked. At the \end,
null boxes will be ejected until the current page and recent contributions
are empty and \deadcycles=0. Thus, you should set \deadcycles=0 yourself
if you have an unusual output routine.

* \insertpenalties is the sum of penalties from split insertions and
floating insertions. \floatingpenalty is the amount added to \insertpenalties
when an insertion is being held over following one that is split.
\insertpenalties is the number of heldover insertions during the time an
\output routine is active.
(All of this should become clear in the new user manual...)
* New integer parameter \globaldefs (is normally zero). If positive, all
definitions are global; if negative, all definitions are local
even when they are prefixed by \global. (Of course, some definitions
are inherently global and will stay that way: \texinfo is global,
and so are things like \prevdepth, \spacefactor, \pagegoal, \deadcycles.)
Example of use: \def\GLOBAL#1{\begingroup\globaldefs=1 #1\endgroup}
allows you to say \GLOBAL\tracingall.

* \mathchoice{D text}{T text}{S text}{SS text}, where the texts are
arbitrary math mode formulas, is now allowed in math mode. TeX will use
whatever text fits the style that eventually governs this part
of the formula.

* \limitswitch is replaced by three primitives \displaylimits, \nolimits,
\limits. The last of these takes the superscript and subscript and puts
them as limits above and below the operator; the first (which is the
default) does this in display style only; \nolimits gives ordinary
subscripts and superscripts. (Now for the first time you can display
limits in superscripts, using \scriptfont3, if you really want to.) To
change old files, you probably need only to replace \limitswitch by \nolimits;
also redefine \int and \oint to include \nolimits.

* \chardef is analogous to \mathchardef: instead of saying
\def\foo{\char<number>} you can say \chardef\foo.
Furthermore, both \chardef and \mathchardef define control sequences
that can be used as integers. For example, the allocation command
\newcount\exno in plain TeX used to produce \def\exno{28 } (for example),
but now it produces \chardef\exno=28. This is more efficient in both
time and space. A \chardef is limited to the range 0..255; \mathchardef
has the range 0..32767.

* \input is treated in the syntax level, so you can say
\expandafter\foo\input bar (in case you want to input a file that
doesn't start with any particular macro). But recursive use
(e.g. \input\input f to get one file name from another) isn't allowed.

* \unbox becomes \unhbox and \unvbox (which are capable of unboxing
only hboxes and vboxes, respectively). Previously you could say
\unbox in either horizontal or vertical mode, but you had to be in
the right mode to match the box; now if you say \unhbox in vertical mode,
or \unvbox in horizontal mode, TeX switches to the correct mode
before doing the unboxing. (The reason the new syntax is needed is that
TeX couldn't otherwise provide this feature without parsing the number
that follows the \unbox, but then when switching to horizontal mode would
have to insert \everypar before the stuff that was already parsed; it
gets pointlessly complicated that way.)

* \unhcopy <number> and \unvcopy <number> are added too; they produce a
copy of the inside of a box. One application is that \struts are now more
efficient: You can define a strut box that contains a \vrule, and
"\def\strut{\unhcopy\strutbox}". Previously, TeX had to scan the
keywords "height" "width" and "depth" and their dimensions whenever it
appended a rule; but rules of width zero make good struts since they take
less memory than boxes. Furthermore, with this new system you can use a
strut to begin a paragraph (even if a box comes first in the paragraph),
instead of saying "\ \unskip" or "\hskip0pt".
(the following changes have been introduced in version 0.96)

* Blank space is no longer ignored after a control sequence name of the form
<escape character><nonletter>. For example, this applies to \/ and \\
and \" and \' and \`. However, any blank spaces after <escape character><space>
are ignored, as part of the general rule that consecutive blanks are treated
as single blanks.

* \crcr acts like \cr except if it appears at the very beginning of an
alignment line; in the latter case it is ignored. Thus, if you're writing
a macro that generates alignment, you can put in an \crcr, which will cover
for users who forgot the final \cr in their argument to the macro.

* Trailing blanks are removed from all input lines. This makes TeX fully
compatible between IBM-like installations (where lines have fixed width)
and DEC-like ones (where lines are terminated by carriage returns).

* The glue above and below displayed equations no longer needs to be the
same. (And it shouldn't be, because a change to \baselineskip inside
the $$'s will affect the top distance but not the bottom one...)
\abovedisplayskip and \belowdisplayskip take the place of \dispskip.
\abovedisplayshortskip and \belowdisplayshortskip take the place of
\dispaskip and \dispbskip.

* \postdisplaypenalty is the penalty for page breaking after a display
(just before the \belowdisplayskip glue). \predisplaypenalty is
the penalty for break before a display (just before the \abovedisplayskip
glue). \displaywidowpenalty is the penalty for breaking between
lines of a paragraph in such a way that there's just one line before a display.

* \tracingrestores=1 will print a symbolic indication of what values
have been restored at the end of a group, on your log file, if
TeX has been compiled with its "stats" switch on. For example,
consider `\lineskip=3pt{\lineskip=4pt.....}'; when the } is sensed,
your log file will say `{restoring \lineskip=3.0pt}'.

* \iftrue and \iffalse are new conditional tests that always go
one way or the other. To use them, you can say, e.g.,
\let\iftitlepage=\iftrue
and later on make tests like
\iftitlepage ... \else ... \fi.
Plain TeX provides a macro \newswitch such that, e.g., \newswitch{titlepage}
defines two new control sequences \titlepagetrue and \titlepagefalse;
the first of these expands to \let\iftitlepage=\iftrue.
You should use this convention instead of saying "\let\iftitlepage=\iftrue"
explicitly in your manuscript, because the "\iftrue" in the latter will
get TeX mixed up if it is skipping over unexecuted conditional text.

* \everyvbox{...} specifies a list of tokens to be inserted just after
the opening left brace of \vbox, \vtop, and \vcenter. Similarly,
\everyhbox{...} specifies a token list for the beginning of \hbox.
One use of these might be to "shut off" macros that were "turned on"
by \everymath and \everydisplay.

* \romannumeral <number> joins \string<cs> and \number<number> and \jobname as
ways to emit text. For example, \romannumeral 324 comes through TeX's
scanner as "cccxxiv" (unless macros are not being expanded);
if \count5=-123, \number\count5 comes through as "-123".
If the number is zero or negative, \romannumeral emits no text.
Some new macros of PLAIN.TEX

The following macros are superior to their "basic.tex" counterparts,
and they are going to be explained in the new manual. Meanwhile,
until you see the new manual, the only way to figure out how to
best use them is to look at the way they are defined in plain.tex...

* Instead of \ldotss use just \ldots, if followed by something
other than a closing delimiter inside a formula; instead of
$\ldotss$, use \dots; instead of $<something>\ldotss<something>$ in
other cases, use "\ldots\,".

* Instead of \twoline, use \displaylines.

* Instead of \chop, use \smash.

* Instead of \cpile, use \matrix.
(the following changes have been introduced in version 0.97)

* Undelimited macro parameters will not be set to "space" unless
you explicitly say "{ }". For example, in \def\a#1#2#3.#4{...},
parameters #1, #2, and #4 are undelimited, while #3 is delimited by a period.
In TeX80, if you said "\a x y z. w" the result was #1=x, #2=space,
#3=y z, and #4=space; this caused confusion. Similarly if you
said \a{...} at the end of a line, you got a space for #2 because
of the carriage return. In TeX82 versions >0.96, you will get
#1=x, #2=y, #3= z (including the space), #4=w.

* \everyjob{<tokens>} specifies something that will be put into TeX's
scanner at the beginning of every job. (This is useful only in a
format that is \dump ed, because otherwise the job has already started!)

* \ifeven disappears; \ifodd<number> takes its place, where <number>
is the number being tested (not the number of a count register).
Thus, where you used to say
\ifeven n alpha \else beta \fi
you now say
\ifodd\count n beta \else alpha \fi

* \if and \ifcat no longer give error messages in cases like "\if\par\let";
unexpandable control sequences are regarded as having character code 256
and category code 16. Thus, you can use \if without fear of an error
stop when the user's input is unexpected.

* \leaders will align from the left of the smallest enclosing box, rather
than from the largest. Thus, for example, if you \moveright a box
that contains leaders, the leaders will move right with it.

* If you say && in the preamble to \halign and \valign, the preamble
will infinitely repeat the material to its right. For example,
\halign{\indent#\hfil&&\hfil#\hfil&#&\cr is like
\halign{\indent#\hfil&\hfil#\hfil&#&\hfil#\hfil&#&\hfil#\hfil...\cr
for as many columns as are actually used. This makes it possible
to write, e.g., a \matrix macro that has no maximum number of columns.
(If you say && twice in the same preamble, the second && is erroneous.)

* \read will read several lines, if necessary, until the number of
left braces equals the number of right braces. "\outer" control sequences
should not appear in the input.

* \uppercase and \lowercase now apply to all character tokens in the token list,
including active characters. (Previously they applied only to characters
of categories 11 and 12.)
(updates to the April 1983 manual)
(these changes were insalled in version 0.98)

* \read n to \cs will not print an explicit prompt message if n<0.
\write n {...} will print only on the log file (not the terminal) if n<0.

* New parameters \hoffset and \voffset will offset the output position.
For example, \hoffset=.5in shifts subsequent output right by half an inch;
\voffset=1.5in shifts it down 1.5in with respect to its normal position.
(The \shipout command adds these offsets to all coordinates of things
that it is shipping out.)

* Here is a fairly major change with respect to TeX macro programs.
The notation for parameters and registers is unified and made more efficient.
1) You don't need \the anymore when using a parameter in an expression.
Thus, \hbox to\hsize works. (So does \hbox to\the\hsize, since a redundant
\the is allowed.) Same for \the\texinfo, \the\lastskip, etc.; \the is
mainly to be used inside \xdef and such things, now. \minusthe disappears.
2) \setcount is eliminated, you just write \count. Similarly \setdimen
and \setskip and \setmuskip are eliminated.
3) \advcount, \advdimen, etc. are now \advance\count, \advance\dimen, etc.;
similarly there is \multiply and \divide. These will work with
parameters as well as registers; e.g., "\advance\abovedisplayskip by 3pt".
4) To compensate for the extra token (\advcount split into two), there
are new commands \countdef, \dimendef, \skipdef, and \muskipdef
(analogous to \chardef and \mathchardef). For example,
\countdef\c=5 makes \c a shorthand for "\count5".
5) "dm" is eliminated as a unit, but instead you can write .5\hsize
or .5\dimen<number>. "vu" and \varunit are also eliminated, as they
are now unnecessary.

The \newcount...\newmuskip macros now give their results in terms of
\countdef...\muskipdef, not \chardef. One consequence is that \dimendef
makes a control sequence behave syntactically like a new dimen parameter.
For example, it would now be possible to say "\newdimen\hoffset" and to
incorporate \hoffset in one's \output routine, thereby making the new
\hoffset parameter unnecessary. Previously, you would have had to tell
users to give values to symbolic dimensions by constructions like
"\def\hoffset{.5in}" because of the incompatibility between register
syntax and parameter syntax. (I apologize for letting this shameful
incompatibility creep in, years ago. Soon it will be gone forever.)

* Here's a simple way to update old macros so that they work with version 0.98:
Case (1), you have not used \newcount, \newdimen, etc.:
Replace \setcount by \count, \setdimen by \dimen, \setskip by \skip,
and \setmuskip by \muskip.
Replace \advcount by \advance\count, \advdimen by \advance\dimen,
\advskip by \advance\skip, \advmuskip by \advance\muskip.
Replace \multcount by \multiply\count, \multdimen by \multiply\dimen,
\multskip by \multiply\skip, \multmuskip by \multiply\muskip.
Replace \divcount by \divide\count, \divdimen by \divide\dimen,
\divskip by \divide\skip, \divmuskip by \divide\muskip.
[That doesn't give the most efficient programs, but it will tide you
over until you have time to rewrite things.]

Case (2), you have used, e.g., \newcount\foo:
Replace \setcount\foo by \foo, \the\count\foo by \the\foo,
\advcount\foo by \advance\foo, \multcount\foo by \multiply\foo,
\divcount\foo by \divide\foo. Similarly for \newdimen et al.

* \everycr{...} inserts its tokens just after TeX has processed \cr or \crcr.
(Except if the \crcr was ignored because it came right after \cr.)

* \hyphenchar=<number> defines the character to be used for
hyphens in that font. If the number is negative or greater than 255,
no hyphenation will be done. Default is '55.

* \skewchar=<number> defines a character to be used to position
accents in math mode. (This makes it unnecessary for a user to refer
to a long, horrible "\skew" table like that in the April draft of the manual!)
When TeX puts a math accent over a character, it shifts the accent to the
right by the amount of kern between that character and the skew character.
Default is -1.

* Furthermore, when a single character is accented, the subscripts and
superscripts of the accented combination are now attached exactly as they
would have been without the accent. (Previously, "\hat A↑2" put the 2 too
high, and "\hat P_2" put the 2 too far away from the P; now both
cases have been fixed.)

* New syntax: \texinfo<number> becomes \fontdimen<number>.
This means that you can define, e.g., \fontspace to be an abbreviation for
"\fontdimen2" and then you can set \fontspace=10pt (analogous to
\skewchar).

* \meaning and \noexpand are new primitives that affect macro expansion:

\meaning<token> expands to the sequence of characters that would be
displayed on the terminal by the existing commands
\let \test = <token> \show\test
For example, "\meaning A" expands to "the letter A" (a sequence of twelve
character tokens, all type otherchar except the spaces). After
\def\A#1B{\C}, "\meaning\A" will expand to "macro:#1B->\C " (a sequence
of fourteen tokens). You can use this in an emergency when \if and
\ifcat and \ifx don't tell you what you need to know about a token.

\noexpand<token> produces the token, but changes its meaning to the
meaning of TeX's \relax primitive if that token would ordinarily
be expanded. Thus, it's now easier to suppress macro expansion;
and you can use \noexpand fruitfully after \if or \ifcat when you
are testing the nature of an unknown token (e.g., a token that
has been found by \futurelet).
Note the following: \catcode`\~=13 \ifcat\noexpand@\noexpand~true\fi
yields "true", but \ifcat\relax\noexpand~true\fi doesn't;
i.e., the TeXbook's conceptual model of token lists, in which an active
character carries category code 13 as a "subscript", is now implemented.

* \afterassignment<token> saves the token and reinserts it into TeX's input
mechanism after the next assignment has been performed. An assignment is a
\def or \let or a command that assigns a value to a control sequence or
internal register; a complete list of assignments will appear in Chapter 24
of The TeXbook. The main intended use of \afterassignment is this: If a the
replacement text of a macro ends with "\afterassignment\continue \dimen0=",
then TeX will assign to \dimen0 whatever <dimen> follows the macro, after
which it will perform \continue. Similarly, you can end with an assignment
that causes TeX to parse <glue> or a font name or even a \parshape.
Changes to PLAIN.TEX conventions since the April manual:

* The \openup macro now takes a dimension argument without braces. It is
recommended to give the amount in terms of a \jot, which plain TeX
sets to 3pt. Thus, where the manual says "\openup{3pt}", you now say
"\openup 1\jot" or "\openup\jot"; where it says "\openup{-3pt}",
you now say "\openup -\jot"; and fractional amounts like "\openup
.5\jot" are legit.

* The character for "ties" between words has been changed from @ to ~. The
next generation of fonts will have an at sign in the normal ascii place,
so @ will not be mentioned specifically as a special character in the
TeXbook. But for the time being (until we have the new fonts), you still need
to say \@ to get an at sign from the math symbols font. Some day there won't
be an at sign in that font, but you'll have bold and italic at signs in
your bold and italic fonts!

Incidentally, although this doesn't affect users, PLAIN and LATEX and AMSTEX
will be using @ as a letter in the names of control sequences that aren't
supposed to be easily redefinable.

* In math formulas, " is no longer used for double prime; the notation is
f''(x) instead of f"(x), and f'''(x) will also work. In other words, " is
now unconstrained, while a sequence of n apostrophes is converted into
↑{\prime...\prime} (there are n occurrences of \prime).
(changes installed in version 0.99)

Version 1.0 is almost here! Just a few more touchups...

* "wd", "ht", and "dp" are changed to \wd, \ht, and \dp. (This is something
like the recent change where "dm" was changed to \dimen.) That means you
don't have to say "1wd0" to get the width of \box0, you just say "\wd0".
Furthermore, you can now assign new dimensions to a box, instead of
relying on trickery: For example, \ht0=2pt changes the height of \box0
to 2pt, independent of what the height was formerly. (But it has no
effect if \box0 is void. The prefix \global is ignored if you happen
to say \global\ht0=2pt.)

* \tokens is being generalized to a set of 256 registers called \toks0
through \toks255. And there is "\toksdef". Plain TeX will contain
a \newtoks allocation macro, of course. (Thus you get symbolic names
for lists of tokens that are unlike macros since you can insert
them into edefs and messages with one-level expansion only.)
[Note: Say "\toksdef\tokens=0" to make your previous programs work.]

* \insert, \vadjust, and \mark are allowed in restricted horizontal mode
and in math mode. However, they won't always migrate outside their boxes
into the main vertical list; migration from an hbox happens only if the
hbox was typeset on the outer level of a vertical list (most uses of
\centerline fit this description) or if the hbox was formed for an
\halign entry. Migration from math mode happens only if the item is on
the outer level of a math formula (not in a subformula) and if the formula
is displayed or if it appears in a paragraph or hbox that allows
migration. Otherwise you to unbox them if you really want to use them.
This makes \hbox and \vbox work approximately the same.

* \font is now acceptable as a font identifier in contexts like
"\fontdimen6\font" and "\hyphenchar\font" and "\textfont0=\font".
It denotes the current font.
Version 0.999

* \leaders in a horizontal list now have height and depth; in a vertical
list they have width.

* the word "by" is optional in the \advance, \multiply, \divide commands.

* \font\foo=name scaled n says to load the font at n/1000 times its
design size. Plain TeX has a macro \magstep<n> such that
\magstep0=1000, \magstep1=1200, \magstep2=1440, \magstep3=1728,
\magstep4=2074, \magstep5=2488, others undefined.
Also \magstephalf=1095. We will make and distribute fonts that are magnified
in such steps.

The \magnify macro has been renamed \magnification and it is designed to
to be used without braces:
\magnification 1200 or \magnification=1200 or \magnification \magstep1
all are equivalent. Note that if you use fonts only at \magstep0 or
\magstephalf, (e.g., \font\ff=cmr7 scaled \magstephalf) you can say
\magnfication\magstephalf and you will still stay in the existing font family.

\magstep 0 leaves 10pt as 10pt;
\magstephalf makes 10pt into 11pt, (very nearly);
\magstep 1 makes 10pt into 12pt;
\magstep 2 makes 10pt into 14pt, (a teeny bit more, actually);
\magstep 3 makes 10pt into 18pt, (a teeny bit less, actually);
\magstep 4 makes 10pt into 21pt, (a teeny bit less, actually);
\magstep 5 makes 10pt into 24pt, (a teeny bit more, actually).

We will make the sixteen fonts of plain TeX available in seven magnifications
(including \magstep 0, of course!). Other fonts will be made in four
magnifications. Any font can, of course, be made in any magnification,
subject to economic considerations.

* New parameters to free TeX from its character set a bit more:
\escapechar is the character inserted before control sequence names by
\write and \string, and when TeX display token lists
\defaulthyphenchar and \defaultskewchar, the values assigned to
\hyphenchar and \skewchar when a font is loaded
\endlinechar, the character placed at the end of an input line.
In the case of \escapechar and \endlinechar, no character is used
if the value is negative or greater than 127.
Normally \escapechar=`\\, \defaulthyphenchar=`-, \defaultskewchar=-1,
and \endlinechar=`\↑↑M (<return>).

* A new tokenlist parameter \errhelp is added. TeX will do the
equivalent of \immediate\write16{\the\errhelp} if the user types "h"
in response to an \errmessage stop. Note: To conserve TeX's memory space,
you should do something like the following [plain TeX will provide a macro]:
\edef\next{\csname This is my help message.\endcsname}
\errhelp=\expandafter{\next}
Then the \errhelp token list is only one token long, and when you write it
TeX will print the long control sequence name like this:
\This is my help message.
The control sequence name goes into TeX's compact string memory, where
TeX's own help messages are stored. By contrast, if you had said
\errhelp={This is my help message.}
you would have taken up space at the rate of one token per letter!

* When TeX writes a token list to a file and the character \newlinechar
occurs, TeX will start a new line instead of writing that character.
(Thus, for example, you can give two-line help messages.)
Plain TeX will set \newlinechar=-1, ignoring this feature.

* Assignments like \toks2=\toks4 and \everypar=\everymath are now allowed.

* When \edef and \message etc. are expanded, expansion is no longer
inhibited after \def; \noexpand should be used to suppress expansion there.

* \the is now treated as an expandable command; it still expands only
one level in \edef and \mark and \write, etc. If you now say, e.g.,
"\hbox to\the\hsize" you lose lots of time w.r.t. "\hbox to \hsize",
because the former expands \the\hsize to a sequence of tokens and
parses them, while the latter has instant access to \hsize.
Conversely, \the is more efficient than \number in \edef,
because it avoids trying to expand the digits after they are generated.
Previous features \the\the and \the\tenrm are no longer allowed;
there's a new command \fontname which expands to the name
of the specified font. (The rules for macro expansion are now
somewhat cleaner and more consistent, and the formal syntax is now nicer.)

* \unhbox and \unhcopy are now allowed in math mode provided that
the box in question is void. This means that you can say \unhcopy\emptybox
in any mode, and it puts you into horizontal mode if you were in
vertical mode. (This is more efficient than \ \unskip.)

* \aftergroup<token> puts the token into TeX's input after the current
group ends. If several \aftergroup commands occur in the same group,
they will be performed in order; e.g.,
{\aftergroup\a \aftergroup\b} yields \a\b.
[This is the historic and climactic final extension to TeX.]
Changes after July version of TeXbook

* There's no longer a blank line inserted at the end of an \input file.
The paragraph at the bottom of page 47 of the manual has been changed to:

[danger][danger] If \TeX\ has nothing more to read on the current line, it
goes to the next line and enters state $N$. However, if \endinput has been
specified for a file being \input, or if an \input file has ended, \TeX\
returns to whatever it was reading when the \input command was originally
given. \ (Further details of \input and \endinput are discussed in
Chapter~20.)