Functions:
   PTR-TYPEP 		Primitive TYPE name for a datum -- see MC:NIL;NEWFUN >
		for specifics.  Currently, PTR-TYPEP is the same as the codes 
		actually stored in the "pointer" in the VAX NIL.  
		See *:VAX-PRIMITIVE-TYPES below.
   EXTENDP 		
   SI:EXTENDP	A basic primitive type is the EXTEND, which is just a chunk 
		of index-access components, and a link to a descriptor object 
		which is itself another EXTEND.  In fact, these "descriptor 
		objects" are essentially CLASSes in the LISPM and SMALLTALK 
		sense.  In MacLISP, they are emulated with hunks.
   EXTEND-LENGTH
   SI:EXTEND-LENGTH	The argument is an "instance" and this function just
		 counts the number of components in that instance.
   SI:MAKE-EXTEND	Takes two arguments -- first is CLASS object for which 
		an instance is being created, 2nd is number of "slots" to be 
		allocated in the instance.
   SI:EXTEND	First arg must be a CLASS -- remaining args are constructed 
		into an object of that CLASS; (SI:EXTEND SOME-CLASS 'T () )
		would cons up a "SOME", and (SI:XREF frob 0) would get the 'T.
   SI:XREF 	basic accessor for EXTENDs, e.g. (SI:XREF <ob> <index>)
   SI:XSET	basic updator, e.g. (SI:XSET <ob> <index> <val>)
   CLASSP  	A predicate which is "true" iff its arg is not only an EXTEND,
		but a class descriptor for some class of objects;  such an
		object, a CLASS object, will henceforth just be called a CLASS.
   CLASS-OF 	For any argument, will return the CLASS for which that argument
		is an instance;  arguments of "primitive" type (i.e., not a
		EXTEND) will be classified according to a systemic table,
		whereas an EXTEND itself will have a direct link to its CLASS.
   SEND		Takes two or more arguments;  the first is an object to
		be acted upon, the second is a "keyword" which will be used
		to select a method from among those accessible in the CLASS
		of the first argument, and the remaining arguments are
		just additional arguments to the method function.
   SEND-AS	Takes three or more arguments.   Second arg is some object
		to be acted upon (third is "keyword") just as with SEND, but 
		first argument is a CLASS which is to be the root for method 
		searching rather than the the class of the active object.
   ADD-METHOD	Takes args -- (<keyword> <fun> <class>) -- and stores the
		function <fun> as a <keyword> method in the CLASS <class>.  
		Thus if SEND sends a message with the key <keyword> to an 
		object in the CLASS <class> (or to any object whose CLASS has 
		<class> as a superior class), then <fun> will be invoked.
		There is an as-yet undocumented means for specifying the
		method-searching function on a class by class basis.
   FIND-METHOD	  (FIND-METHOD <msg> <class>) Returns the function 
		which will handle message <msg> for the CLASS <class>.
   DESCRIBE	Prints a brief description of the object;  for EXTENDed 
		objects, retrieves some information from the corresponding 
		CLASS;  for CLASS's themselves, will also do the DESCRIBE 
		operation for all its superiors.
   WHICH-OPERATIONS	For any object as argument, will print out a sequence 
		of lines with the (1) name of each "operation" (or message) 
		which is applicable to this object (as well as any object in 
		the same CLASS), (2) the name of the function which will handle
		that operation, and (3) the name of the CLASS from which that 
		method was "inherited." See also SI:OPERATIONS-LIST below.
   TYPE-OF 	Retrieves the TYPE name as stored in the CLASS for the datum 
		given as argument.  For all MacLISP primitive datatypes (like,
		SYMBOLs, BIGNUMS, etc), this will be the same as the TYPEP of 
		the datum.
   EQ-FOR-EQUAL? 	Takes two args -- returns "true" only if EQ is 
		satisfactory for the EQUAL test (e.g., as on SYMBOLs).

   SI:DEFCLASS*-1   Argspectrum is like (<typename> <class-var> <suprs>
					   &optional (<class-name> <typename>))
		Normally, called by using the DEFCLASS* macro; 
		<typename> is a symbol to be used to describe the "type" of 
		    objects and is stored in a slot of the CLASS retrieveable 
		    by SI:CLASS-TYPEP and TYPE-OF (see below).
		<class-var> is a symbol which will be set to the CLASS about 
		    to be created; conventionally, if FOO is chosen as the 
		    type name, then FOO-CLASS is the class variable.
		<suprs> a list of CLASSes which are to be considered superiors 
		    of this class (for the purposes of inheritance of 
		    properties, such as "methods").
   SI:HAS-SUPERIOR	A predicate, "true" iff the second arg is some super-
		class of the first arg.  Both arguments must be classs.
   SI:WHERE-IS-METHOD   As above with SI:FIND-METHOD, but returns the actual 
		CLASS under which that function was stored as a method.
   SI:OPERATIONS-LIST 	Argument must be a CLASS;  returns a list of items 
		like  `(,<key> ,<fun> ,<class>),  where <key> is a method 
		name (or message key word), <fun> is the function which 
		handles that method, and <class> is the CLASS in which it was 
		found.  <class> will either be the same as the argument, or 
		one of the superiors of the argument.


The following functions (which are SUBR's) also have macro definitions
which are "enabled" when compiling, to produce more efficient compiled code:
   EXTEND-LENGTH,  EXTENDP (merely produces call to the "SI:"-named versions)
   SI:MAKE-EXTEND, SI:EXTEND,  SI:XREF,  and  SI:XSET
Macros (only):
   DEFCLASS*	(DEFCLASS* <name> <variable> <superiors> . <options>) creates 
		a new CLASS named <name>, and putting it in the variable named 
		<variable>;  the superior class may not be ommited.  Typical 
		use might be
		(DEFCLASS* MUMBLENUM MUMBLENUM-CLASS (NUMBER-CLASS))
		[Note that the value of OBJECT-CLASS is the CLASS which is the 
		 root of the CLASS hierarchy].
   DEFMETHOD*	Defines a method for a particular class, to be used when a 
		specified message is sent.  For example,
			(DEFMETHOD* (KEY FOO-CLASS) (OBJ . <arg-vars>) 
				. <body>)
		defines a KEY method for instances of class FOO.  Then when 
		(SEND <bar> 'KEY <a1> <a2>) is executed, OBJ is bound to <bar>,
		the <arg-vars> are bound (or "spread-out") to <a1> and <a2>, 
		and <body> is executed.  KEY can be a list of keys instead of 
		a single key.
   TYPECASEQ	Like CASEQ, but dispatches on the "primitive" type of the 
		datum.  E.g.,  (TYPECASEQ X ((SYMBOL STRING) 5) (T 6))
		could be emulated by (CASEQ (PTR-TYPEP X)
					    ((SYMBOL STRING) 5) 
					    (T 6))
		In some implementations, however, this will be done by a 
		fast dispatch through a table, rather than by sequence of
		EQ comparisons.
   MAPF		A generalized Mapper, usable on VECTORs, FIXNUMs, as well as 
		lists.  See MC:NIL;NEWFUN > for more details.
   **SELF-EVAL**	Most EXTENDed data will have this symbol stored in
		the (CXR 1 <obj>) slot.  The macro-expansion of 
		(**SELF-EVAL** ...) yields (QUOTE (**SELF-EVAL** ...))
   **CLASS-SELF-EVAL**  Same as for **SELF-EVAL**, but used only in CLASS's -- 
		this provides a simple heuristic to distinguish them quickly 
		from other hunks, and other EXTENDs
   SI:EXTEND-CLASS-OF	1 arg.  If object is know to be an EXTEND, this will
			produce much faster code than calling CLASS-OF.
   SI:CLASS-VAR
   SI:CLASS-NAME 
   SI:CLASS-TYPEP
   SI:CLASS-SUPERIORS  Each CLASS (an EXTENDED data object which serves as 
		a descriptor for a whole class of objects) has about 17. 
		slots of data, the 5 more important of which are listed here
		by the accessor macro names.  Typically, each class has some
		symbolic name, and some global variable whose value is that
		class object; e.g. let <v> be the CLASS for VECTORs, then
		    (SI:CLASS-NAME <v>) ==> VECTOR,         and
		    (SI:CLASS-VAR <v>)  ==> VECTOR-CLASS
		As a general convention, any class named FOO has the
		class variable FOO-CLASS.  The SI:CLASS-TYPEP component
		is the value to be returned by TYPE-OF. SI:CLASS-SUPERIORS
		is a list of CLASS's of which this one is a subclass;
		primarily, the superiors are of interest when searching
		for applicable methods, and the search goes "upwards" in
		order to "inherit" properties from the superiors.

Variables:
   NULL-CLASS 			CLASS for () 
   PAIR-CLASS			CLASS for non-null lists
   and CLASS's for standard MacLISP data types:
     FIXNUM-CLASS, FLONUM-CLASS, BIGNUM-CLASS, SYMBOL-CLASS, ARRAY-CLASS
     HUNK-CLASS, SFA-CLASS, FILE-CLASS, JOB-CLASS, RANDOM-CLASS
   *:TRUTH 		In MacLISP world, this is just "T", but in a real 
			NIL, it will be a new, unique constant to mean 
			"true";  "false" will still be represented by ().
   *:VAX-PRIMITIVE-TYPES 	A list of SYMBOLs representing the names
			of the data types whose type codes are stored in
			in the "pointer" itself, for the VAX implementation 
			of NIL (i.e., not embedded as EXTENDs)
   SI:INITIAL-CLASSES  Holds a list of lists -- total of 22. -- each list
		of which holds a little information about the initial
		set up of the CLASS system;  (mapcar 'CAR SI:INITIAL-CLASSES)
		returns a list of the names of all initial CLASSes.

Error facilities:
    Because the EXTEND facilities are designed to provide the MacLISP
user with some of the amenities of NIL or the LISPM, some extension
to the error system was needed, and it comes from the system file
LISP:CERROR --  CERROR and FERROR are supported, as on the LISPMachine, 
but FORMAT is not forcibly loaded (it is called only when already loaded 
so that one may use EXTENDs without having to load the humungous FORMAT file).
A few global variables are set up:
    STANDARD-OUTPUT is set to T,  and
    ERROR-OUTPUT is set to a SFA which directs output to MSGFILES 

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