The simplest solution (from the implementor's perspective) is probably to leave it to the user to reassign one. Allowing multiple-letter specifiers helps here. Coming from a C perspective, that's what I would prefer.

You could work out all sorts of namespacing and automatic reassignment schemes, of course.

I can't think of anything without trade-offs off the top of my head (I double that anything exists), but in my (limited) experience it's very workable.