Will probably try other approaches soon.

Strangely, this slows things down and does not always save memory (!?).

(See patch in attic/word_and_lookahead.patch.)
This consistently saves 10% or 15% in space but costs just as much
in time, probably because hash-consing sees many more unique strings
(about 30x).

and performance of a new version with respect to the previous version.

This makes almost no difference: it saves 5 nodes out of 25K in
the stars, and saves 10K facts out of 7M facts (on ocaml.mly --lalr).

Not essential. Perfectionism, I suppose.

in a binary search tree.

No speed gain, but this allows removing [MyMap].

not sure why.