#[global]
Instance filter_ok s f `(Ok s) : Ok (filter f s).
(** ** Partition *)

Lemma INV_lt x1 x2 l1 l2 acc :
INV (x1 :: l1) (x2 :: l2) acc -> X.lt x1 x2 ->
INV (x1 :: l1) l2 (x2 :: acc).