>> 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 ;-). Regards, Bruno