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 nth_error_succeeds : forall {A} n (l : list A) (Hlt : (n < length l)%nat),
exists x, nth_error l n = Some x.