🚀 go-pugleaf

In article <ORLOFF.92Oct29192755@surya4.cern.ch>, orloff@surya4.cern.ch (Jean Orloff) writes...
>Let me rephrase the heart of my problem: suppose I have a control
>sequence \text containg a_11, b_11, {_1, c_11, }_2. How would I change
>the catcodes of the inner braces???

I don't see how this helps one redefine \input.  But anyway...

%  Here is a way to re-catcode a macro to "other."
%  If there are \'s in the macro replacement text, there might be 
%  some extra space inserted (\that) -> (\that ).
%  It goes to some pain to fix doubled # signs.

% \recatcode takes a single macro as its parameter and redefines that
% macro so its replacement text is all catcoded "other"

{\catcode`\@=11

\gdef\@meaningless#1:->{\@undoublehash}%

\gdef\@gobblethree#1#2#3{}

\catcode`\&=6 \catcode`\#=12 % use & for macro parameters while # is "other"

\gdef\recatcode&1{%
  \toks@=\expandafter{&1}\edef\n@@t{\the\toks@}% prevents error message with #
  \edef&1{\expandafter\@meaningless\meaning\n@@t#\@gobblethree#}}%

\gdef\@undoublehash &1#&2#{&1%
  % if ## emit #\@undoublehash
  % if #\@gobble# emit nothing
  % if #other# emit #other#\@undoublehash (can't happen(?))
  \ifx&2\@gobblethree\@gobblethree #\expandafter\@undoublehash
  \else #&2#\expandafter\@undoublehash\fi} 

}

\tracingall

\def\text{ab{c}} \tt \text
\recatcode\text \text.

\bye    Donald Arseneau