(*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 decompose_Node: forall t1 t2 c c1 c2, decompose (exist _ (Node t1 t2) c) = (exist _ t1 c1, exist _ t2 c2).