TODO note.

TODO

+* Is it true that the warning "this production is never reduced" is
+  sound but incomplete? i.e. certain states could be unreachable and
+  we do not know it (but LRijkstra can tell us).
+  Take never_reduced.mly and see if we can have a production that is
+  never reduced *and no warning about it*. (Just place it behind
+  another production that is never reduced.) Document this fact.
+
 * Another crazy idea: if we had a special "void" non-terminal symbol,
   we could use it as a parameter, which would allow us to make extensible
   definitions. (Any production that contains void would be removed after