proposition: Lemma right_cancel_from_left `{!Commutative op} `{!LeftCancellation op z} : RightCancellation op z.
proof: Proof. intros x y E. apply (left_cancellation op z). rewrite 2!(commutativity (f:=op) z _). assumption. Qed.
imports: HoTT.Classes.theory.groups HoTT.Classes.theory.apartness HoTT.Classes.interfaces.abstract_algebra