(** ** Subset *)

Lemma subsetr_spec : forall subset_r1 r1 x1 c1 s2,
bst (Node c1 Leaf x1 r1) -> bst s2 ->
(forall s, bst s -> (subset_r1 s = true <-> Subset r1 s)) ->
(subsetr subset_r1 x1 s2 = true <-> Subset (Node c1 Leaf x1 r1) s2).