Am 12.05.2015 um 16:34 schrieb Joseph Wright:

>
> Probably we should add something: as Ulrike says, at the very least
> \prop_show should complain.

we probably should, but I don't think that \prop_show:N would be the 
right place. True, some other "prop" functions do seem to work if you 
start from a "c" generated name rather than with \prop_new:N but this is 
at best by chance: they are defective and who knows what other defects 
are hidden or appear if further "prop" functions are being written.

So despite the time penalty I think a quick check always that an 
argument that should be a "prop" is not equal to \relax is safer.

frank