intros x y z H' H'0; try assumption.
apply Trans with (ring_mult (ring_unit K) y).
auto with algebra.
apply Trans with (ring_mult (ring_mult (field_inverse x) x) y).
apply Sym; auto with algebra.
apply Trans with (ring_mult (field_inverse x) (ring_mult x y)).
auto with algebra.
apply Trans with (ring_mult (field_inverse x) (ring_mult x z)).
auto with algebra.
apply Trans with (ring_mult (ring_mult (field_inverse x) x) z).
auto with algebra.
apply Trans with (ring_mult (ring_unit K) z); auto with algebra.
Qed.

(* Specifications: *)
Lemma FIELD_reg_right :
forall x y z : K,
~ Equal x (monoid_unit K) ->
Equal (ring_mult y x) (ring_mult z x) -> Equal y z.