(*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_height_zero: forall s sL sR,
decompose s = (sL,sR) ->
height s = 0 ->
sL = s /\ sR = s.