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 Znth_In : forall A i (l : list A) d x (Hrange : 0 <= i < Zlength l)
(Hnth : Znth i l d = x), In x l.