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 Forall_firstn : forall {A} P (l : list A) n, Forall P l -> Forall P (firstn n l).