>> Of course, the <value> must be a token list, but the <key> could be
>> more restricted?
> I thought we were talking about sequences! (I suggest we finish that
> discussion before moving on to other things.)
Yes, I'll look at the seq code very soon. I was just pushing forward
the natural continuation of Frank's question on an expandable
\prop_if_in:NnTF test. If catcodes matter, then it is impossible to
provide an expandable test.
>> Here <values> may be anything, but <keys> are rather well behaved. The
>> key (sic) property of <keys> in my understanding is whether they
>> differ or not, and detokenizing will not cause too many collisions?
> Within xtemplate, you'll find some places where we use the fact that the
> keys do have catcodes to pull off some 'special effects'. Now, we also
> remove spaces there, so spaces can also be used for said 'special
> effects'. But such a change would require some alterations.
Ok, so my assumption was wrong. But really, playing with catcodes is evil ;-).