(** Two-argument functions that allow to reorder their arguments. *)
Definition transpose (f : A -> B -> B) :=
  forall (x y : A) (z : B), eqB (f x (f y z)) (f y (f x z)).(** A version of transpose with restriction on where it should hold *)
Definition transpose_restr (R : A -> A -> Prop)(f : A -> B -> B) :=
  forall (x y : A) (z : B), R x y -> eqB (f x (f y z)) (f y (f x z)).

Lemma fold_right_commutes : forall s1 s2 x, eqB (fold_right f i (s1++x::s2)) (f x (fold_right f i (s1++s2))).