WISHLIST:

Allow backtracking in the generated parser.

It would be nice if a symbol which is not declared and has no rules for it
is implicitly declared to be a terminal with a warning.  Currently it is
implicitly declared to be a nonterminal resulting in an error.

Redo the frontend, taking care of the above and getting rid of the wall
between the grammar frontend and the named attributes frontend.

Bison seems to allow %type to be used to declare the types of a terminal
symbol, even tho' the documentation is quite specific that %type is used
only for nonterminals.  Maybe zyacc should allow that too.
