Section Fold_More.
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).
(** ** Fold is a morphism *)
(** ** Fold and other set operators *)
End Fold_More.
End Fold.
(** * Cardinal *)
(** ** Characterization of cardinal in terms of fold *)
(** ** Old specifications for [cardinal]. *)

Lemma cardinal_2 :
forall s s' x, ~ In x s -> Add x s s' -> cardinal s' = S (cardinal s).