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.

Lemma fold_4 : 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 -> Below x s -> Add x s s' -> eqA (fold f s' i) (fold f s (f x i)).