Bruno Le Floch skrev 2015-08-27 12.34: > That's much harder. Catcode changes are probably warranted in this > case, since teaching TeX to nest parentheses in this way is tough. I seem to recall that I did demonstrate how to do that -- have TeX match parentheses in an expression, using delimited arguments rather than catcode changes -- in a mail to this very list several years ago... Yes, it was on 2011-10-19 and the subject was "Re: Church booleans". The attachment with the full implementation was included in the posting from the list, so I hope it also made it into the list archives. (But I don't really have the energy at the moment to figure out what my password is so that I can check.) Lars Hellström PS: I maintain that that approach to evaluating infix boolean expressions is far superior to what is currently offered in LaTeX3, but Bruno just seems to *love* his &&, with all its shortcomings. ;-)