(*D11*)
Definition sub (x y : canonTree) : option canonTree :=
match eq_dec (glb x y) y with
| left _ => Some (glb x (comp y))
| right _ => None
end.

Lemma tree_case_rewrite : forall t,
(t = top \/ t = bot) \/ exists t1 t2 c, t = exist _ (Node t1 t2) c.