This removes the dependency of [Interface] on [PreFront].

Explicitly parameterize [TokenType] over a grammar.
This removes the dependency of [TokenType] on [PreFront].

So we will find out if anything changes in the warnings and errors
that menhir produces.

bench/good: added the known bug cutdown.mly.

or the singleton set {epsilon}.

This grammar used to contain a start symbol that accepts the empty word!

files -- it was apparently dead, since all .expected files are here.

Currently equal to the performance of the table back-end.