Section Fold.
Variables (A:Type)(eqA:A->A->Prop)(st:Equivalence eqA).
Variables (f:elt->A->A)(Comp:compat_op E.eq eqA f)(Ass:transpose eqA f).
Variables (i:A).
Variables (s s':t)(x:elt).

(* Specifications: *)
Lemma fold_empty: (fold f empty i) = i.
Lemma fold_equal:
equal s s'=true -> eqA (fold f s i) (fold f s' i).
Lemma fold_add:
mem x s=false -> eqA (fold f (add x s) i) (f x (fold f s i)).
Lemma add_fold:
mem x s=true -> eqA (fold f (add x s) i) (fold f s i).
Lemma remove_fold_1:
mem x s=true -> eqA (f x (fold f (remove x s) i)) (fold f s i).
Lemma remove_fold_2:
mem x s=false -> eqA (fold f (remove x s) i) (fold f s i).
Lemma fold_union:
(forall x, mem x s && mem x s'=false) ->
eqA (fold f (union s s') i) (fold f s (fold f s' i)).