This is TeX, Version 2.2 (INITEX)  14 JUN 1987 17:38
**t
(t.tex (basic.tex)
| \par =\par
| begin-group character {
| \def =\def
| \t =undefined
| begin-group character {
| \gdef =\gdef
| \a =undefined
| macro parameter character #
| macro parameter character #
| end-group character }
| \catcode =\catcode <i
 | the character `
 | the letter d
 | the character =>
| the character = <i
 | the character 1
 | the character 2
 | \t =macro
 | \gdef =\gdef>
| \gdef =\gdef
| \a =undefined
| macro parameter character #
| the character 1
| the character d
| macro parameter character #
| the character 2
| macro parameter character #
| the character 3
| begin-group character {
| macro parameter character #
| the character 2
| end-group character }
| end-group character }
| blank space  
| \par =\par
| \hfuzz =\hfuzz
| the character = <d
 | the character 1 <i
  | the character 1
  | the character 0
  | the character 0
  | the letter P>
 | the letter P <'em'
  | the letter P> <'ex'
  | the letter P> <'true'
  | the letter P> <'pt'
  | the letter P
  | \ifdim =\ifdim <x <d
    | the character 1 <i
     | the character 1
     | the character 2
     | the letter p>
    | the letter p <'em'
     | the letter p> <'ex'
     | the letter p> <'true'
     | the letter p> <'pt'
     | the letter p
     | the letter t*>
    | the character =>
   | the character = <d
    | the character 1 <i
     | the character 1
     | the letter P>
    | the letter P <'em'
     | the letter P> <'ex'
     | the letter P> <'true'
     | the letter P> <'pt'
     | the letter P
     | \expandafter =\expandafter <x
      | \a =macro
      | \par =\par>
     | \a =macro
     | \par =\par
Runaway argument?
! Paragraph ended before \a was complete.
<to be read again> 
                   \par 
l.8 
    
? h
I suspect you've forgotten a `}', causing me to apply this
control sequence to too much text. How can we recover?
My plan is to forget the whole thing and hope for the best.

? x
 
Here is how much of TeX's memory you used:
 35 strings out of 1840
 183 string characters out of 9427
 1053 words of memory out of 3000
 327 multiletter control sequences out of 2100
 5687 words of font info for 19 fonts, out of 20000 for 75
 0 hyphenation exceptions out of 307
 2i,0n,0p,83b,8s stack positions out of 200i,40n,60p,500b,600s
No pages of output.