Stylistic question about inheritance

[Lonnie Princehouse]

Well, that's true, but I meant to convey that no grammatical entity is
the base class of another entity, so it's a flat inheritance tree in
that respect.  ASTNode would not be something that the parser would
know anything about.

I guess that's sort of moot if your expression trees are just a
contrived example; in that case, I'd say that how deep you want your
inheritance hierarchy to be depends entirely on how your program wants
to use it.