(*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_right_obmit: forall a a1 a2 t1 t2 c c1,
t1 <> Leaf false ->
decompose a = (a1,a2) ->
unrel (exist _ (Node t1 t2) c) a = unrel (exist _ t1 c1) a1.