(** ** Subset *)

Lemma subsetl_spec : forall subset_l1 l1 x1 c1 s2
`{Ok (Node c1 l1 x1 Leaf), Ok s2},
(forall s `{Ok s}, (subset_l1 s = true <-> Subset l1 s)) ->
(subsetl subset_l1 x1 s2 = true <-> Subset (Node c1 l1 x1 Leaf) s2 ).