(*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.