(*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 unrel_lub: forall a b1 b2, unrel a (lub b1 b2) = lub (unrel a b1) (unrel a b2).