(*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_Node: forall t1 t2 c c1 c2,
decompose (exist _ (Node t1 t2) c) = (exist _ t1 c1, exist _ t2 c2).