> In fact one could think of also having
> \<thing>_case:nnTF
> where the T branch is selected if you have a match (in addition to 
> executing any match code from the "n" argument). 
This sounds more logical for me and I really like this idea. I think the 
true part is mostly a message instead of a function. However for me as a 
beginner it's more consistent in a expl3 manner. ;-)

So after this small discuss I vote for the following:


Best regards