Bruno Le Floch writes:
> > \foo [arg] has the standard 'argument meaning'
> > \foo
> > [Paranthetical text starts here. ...
> The problem is that the two are treated identically by TeX, except if
> catcodes have not yet been assigned, and one changes the catcode of
> eol or space.
or alternatively building out the command names yourself by putting letters on
catcode 13. that might be in fact an easier approach and then explicitly
deciding when to gobble for spaces
> Proposal: to ban trailing optional arguments entirely.
well we can't do that with document level dtd's that provide some level of
backward compatibility and for xparse I wouldn't attempt for formally ban it
(though any discussion about use of optional arguments should make it clear
that putting them at the end is not a good idea in general).