Section FoldOpt.
Variables (A:Type)(eqA:A->A->Prop)(st:Equivalence eqA).
Variables (f:elt->A->A)(Comp:Proper (E.eq==>eqA==>eqA) f).

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