(** The following results have already been proved earlier, but we can now prove them with one hypothesis less: no need for [(transpose eqA f)]. *)
Section FoldOpt.
Variables (A:Type)(eqA:A->A->Prop)(st:Equivalence eqA).
Variables (f:elt->A->A)(Comp:compat_op E.eq eqA f).

(* Specifications: *)
Lemma fold_equal :
forall i s s', s[=]s' -> eqA (fold f s i) (fold f s' i).
Lemma add_fold : forall i s x, In x s ->
eqA (fold f (add x s) i) (fold f s i).
Lemma remove_fold_2: forall i s x, ~In x s ->
eqA (fold f (remove x s) i) (fold f s i).