Variable X: Set.
Variable Xle: X -> X -> Prop.
Hypothesis XO: preorder X Xle.
Definition Xlt (x y: X): Prop := Xle x y /\ ~ Xle y x.

Lemma Xle_nlt: forall x y, Xle x y -> ~ Xlt y x.
Inductive sorted: forall n, Vector.t X n -> Prop := (* Coq.Sorting's definition sucks *)
| sorted_nil: sorted Vector.nil
| sorted_one x: sorted (Vector.cons x Vector.nil)
| sorted_more (a b: X) n (t: Vector.t X n):
Xle a b -> sorted (Vector.cons b t) -> sorted (Vector.cons a (Vector.cons b t)).