(*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_basic: forall b c c1 c2,
decompose (exist _ (Leaf b) c) = (exist _ (Leaf b) c1,exist _ (Leaf b) c2).