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. ;-)