Section Axiomatisation.
Variable U : Type.
Variable op : U -> U -> U.
Variable cong : U -> U -> Prop.
Hypothesis op_comm : forall x y:U, cong (op x y) (op y x).
Hypothesis op_ass : forall x y z:U, cong (op (op x y) z) (op x (op y z)).
Hypothesis cong_left : forall x y z:U, cong x y -> cong (op x z) (op y z).
Hypothesis cong_right : forall x y z:U, cong x y -> cong (op z x) (op z y).
Hypothesis cong_trans : forall x y z:U, cong x y -> cong y z -> cong x z.
Hypothesis cong_sym : forall x y:U, cong x y -> cong y x.
(** Remark. we do not need: [Hypothesis cong_refl : (x:U)(cong x x)]. *)

(** Needed for treesort ... *)
Lemma twist :
forall x y z t:U, cong (op x (op (op y z) t)) (op (op y (op x t)) z).