(** ** Fold is a morphism *)

(* Specifications: *)
Lemma fold_init : forall i i' s, eqA i i' ->
eqA (fold f s i) (fold f s i').
Lemma fold_equal :
forall i s s', s[=]s' -> eqA (fold f s i) (fold f s' i).
Lemma fold_init : forall i i' s, eqA i i' ->
eqA (fold f s i) (fold f s i').
Lemma fold_equal :
forall i s s', s[=]s' -> eqA (fold f s i) (fold f s' i).