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_sorted_lt : forall x i n l lo hi (Hsort : sorted l) (Hlo : lo >= 0)
(Hhi : hi <= Zlength l)
(Hin : In x (sublist lo hi l))
(Hi : lo <= i < hi) (Hn : Znth i l 0 = n)
(Hgt : x < n),
In x (sublist lo i l).