This is TeX, Version 3.1415 (C version 6.1) (format=latex 96.7.6)  19 NOV 1996 21:20
**\makeatletter

*\def\x{\@ifnextchar`\{{\mit}{\ohne}}

*\def\mit#1}{\message{mit #1.}}
! Missing { inserted.
<*> \def\mit#1}
               {\message{mit #1.}}
? 
mit ##1.
*\def\x{\@ifnextchar`\{{{\catcode`\{=12\mit}}{\ohne}}

*\def\x{\@ifnextchar`\{{{\catcode`\}=12\mit}}{\ohne}}

*\def\x{\@ifnextchar`\{{\catcode`\}=12\mit}{\ohne}}

*\catcode`\]=2

*\catcode`\}=12

*\def\mit#1}{\message{mit #1.]]

*\catcode`\}=2

*\catcode`\]=12

*\def\x{\@ifnextchar`\{{\catcode`\}=12\mit}{\ohne}}

*\catcode`\]=2\catcode`\}=12

*\def\mit#1}{\catcode`\}=2\message{mit #1.]]

*\catcode`\]=12\catcode`\}=2

*\x hallo

*\def\ohne#1 {\message{ohne #1.}}

*\x hallo

*\def\x{\@ifnextchar`\{{\mit}{\ohne}}

*\x hallo

*\ohne hallo

*
Runaway argument?
{\ohne }hallo \def \ohne ##1 {\message {ohne ##1.}} \x hallo \def \x \ETC.
! Paragraph ended before \mit was complete.
<to be read again> 
                   \par 
<*> 
    
? 
(Please type a command or say `\end')
*x

! LaTeX Error: Missing \begin{document}.

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<*> x
     
? x
 
Here is how much of TeX's memory you used:
 4 strings out of 10960
 150 string characters out of 73522
 42368 words of memory out of 262141
 2929 multiletter control sequences out of 9500
 3640 words of font info for 14 fonts, out of 100000 for 255
 14 hyphenation exceptions out of 607
 4i,0n,4p,54b,14s stack positions out of 300i,40n,60p,3000b,4000s
No pages of output.