Lemma sorted_dec (s : seq R) x0 (x : R) : sorted Rle s -> head x0 s <= x <= last x0 s -> {i : nat | nth x0 s i <= x < nth x0 s (S i) /\ (S (S i) < size s)%nat} + {nth x0 s (size s - 2)%nat <= x <= nth x0 s (size s - 1)%nat}.