Module Import P := Properties M.
Definition gtb x y := match E.compare x y with GT _ => true | _ => false end.
Definition leb x := fun y => negb (gtb x y).
Definition elements_lt x s := List.filter (gtb x) (elements s).
Definition elements_ge x s := List.filter (leb x) (elements s).
Hint Resolve gtb_compat leb_compat : fset.
Definition Above x s := forall y, In y s -> E.lt y x.
Definition Below x s := forall y, In y s -> E.lt x y.
(** Two other induction principles on sets: we can be more restrictive on the element we add at each step.  *)
(** More properties of [fold] : behavior with respect to Above/Below *)

Lemma fold_3 :
forall s s' x (A : Type) (eqA : A -> A -> Prop)
(st : Equivalence eqA) (i : A) (f : elt -> A -> A),
compat_op E.eq eqA f ->
Above x s -> Add x s s' -> eqA (fold f s' i) (f x (fold f s i)).