I wrote: > A better proposal than using different category codes is > programming macros with optional arguments obeying > a convention that makes a choice of delimiters available to users. > Any macro package replacing \@ifnextchar[ by some > \ifx\let@token\opt@arg@delim could be run this way. > > If no LaTeXY will adopt this, I will call it LüTeX :-) I had thought of posting such a "feature request" earlier, refrained from it for not having found a good idea how to implement it. If I now found it so simple, I must have overlooked a problem ... This would mean that so many \foo in the LaTeX kernel reading their optional argument by \@foo[#1] must be replaced. At least I have now thought how such macros would have to look like instead (sketch, \SetOptArgDels chooses delimiters, \foo is an example command with one optional argument): \def\SetOptArgDels#1#2{% \let\opt@arg@del@l#1% \def\read@pass@opt@arg##1#1##2#2{##1{##2}}} \def\foo{% \@ifnextchar\opt@arg@del@l {\read@pass@opt@arg\@foo}% {\@foo{DEFAULT}}} \def\@foo#1... -- where one had \def\@foo[#1]... so far. Maybe this already is in xparse? Sorry, I cannot afford more right now ... Fortunately, 1. there would be no need to change preamble-only commands, 2. as I realize at reading Heiko's comment, the "ambiguity" only arises with macros whose *last* (e.g., only) arguments are optional. Cheers, Uwe.