(** 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)).