(*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_share_height_no_increase: forall sh sh' sh'' , decompose sh = (sh',sh'')-> height sh' <= height sh /\ height sh'' <= height sh.