(*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 lub_rel_recompose: forall sh1 sh2, lub (rel Lsh sh1) (rel Rsh sh2) = recompose (sh1,sh2).