Fixpoint wf_btree_over t :=
match t with BNode keys children =>
(length keys = 2 * d + 1)%nat /\
(children = [] \/ length children = S (length keys) /\ Forall wf_btree_aux children)
end.

Lemma insert_wf : forall v t t' (Hinsert : b_insert v t = Some t') (Hwf : wf_btree t),
wf_btree t'.