(*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_left_obmit: forall a a1 a2 t c c', decompose a = (a1,a2) -> unrel (exist _ (Node (Leaf false) t) c) a = unrel (exist _ t c') a2.