(*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_basic: forall b c c1 c2, decompose (exist _ (Leaf b) c) = (exist _ (Leaf b) c1,exist _ (Leaf b) c2).