More examples.
Right-align heads of productions in grammar's to_str output.
Initial thoughts on supporting grammar transformations.

Notes on eliminating left-recursion.

Start of trait for symbol generation.
Alpha-rename Sym to ProductionSym.

(Sym is too good a name to give up for internal plumbing.)
Threaded a symbol registry through the grammar constructions so that I
can implement the GenFresh trait.
Plausible implementation of Primable.
Implemented code to eliminate immediate left recursion.
Some predicates on Productions.