(*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_glb: forall a b1 b2,
unrel a (glb b1 b2) = glb (unrel a b1) (unrel a b2).