(*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_join: forall x a b c, join a b c -> join (unrel x a) (unrel x b) (unrel x c).