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

Lemma remove_fold_2: forall i s x, ~In x s ->
eqA (fold f (remove x s) i) (fold f s i).