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 In_Znth : forall A (l : list A) x d,
In x l ->
exists i, 0 <= i < Zlength l /\ Znth i l d = x.