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 sorted_mono : forall d l i j (Hsort : sorted l) (Hi : 0 <= i <= j)
(Hj : j < Zlength l),
Znth i l d <= Znth j l d.