Proper syntax for joining a repeated choice.

[jschwartzentruber]

To do multiple unique syms with a joining string, these patterns are possible:

    (sym ' ')<1,*>

This leaves trailing whitespace, which hopefully doesn't matter. It breaks when the joining string is ,

    sym_a (' ' sym_no_a)<0,*>

Pull one choice out of sym into an always present first term. This may not be desirable.

We should support some syntax for joining a choice together without these compromises.