(*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_left_obmit: forall a a1 a2 t c c',
decompose a = (a1,a2) ->
unrel (exist _ (Node (Leaf false) t) c) a =
unrel (exist _ t c') a2.