Philipp Stephani skrev:
> Am 17.02.2010 um 14:12 schrieb Lars Hellström:
>>> – e.g. with argument specifiers like "s" and "v" in xparse.
>> Not quite that compact, but the concept of "argument processors"
>> was  added to xparse in part to support this kind of thing.

(By "this kind of thing" I meant harmless character sequences, not 
\verb-style arguments.)

> I have read about them, but the manual says "xparse introduces the
> idea of an argument processor, which is applied to an argument after it
> has been grabbed by the underlying system but before it is passed to
> ⟨code⟩." so they can't be applied here – the catcode changes must be
> performed before the argument is grabbed. Once it is through the input
> processor, things like spaces and comments are inevitably lost.

The trick is to not rely on catcode changes, but employ other methods 
for saying what you want. In my experience, and unless you only target 
one thing, you lose more in the corner cases by using \verb and 
\detokenize than you gain in convenience.

Lars Hellström