> Just don't allow 0.

Inevitibly though, 0 will be supported, explicit rule to the contrary
notwithstanding. Someone will say it makes sense, that "0,1" means the last
character.  And once 0 is "supported" there will be confusion about what it
means.   So I would ask, is there really a use case -- not "is it useful"
but "is there a requirement".

--Ray