intros x y z H'; try assumption.
cut
(Equal (ring_mult (sgroup_law R x (group_inverse R y)) z) (monoid_unit R)).
intros H'0; try assumption.
elim (INTEGRAL_DOMAIN_prop (x:=sgroup_law R x (group_inverse R y)) (y:=z));
auto with algebra.
intros H'1; try assumption.
left; try assumption.
apply GROUP_reg_right with (group_inverse R y); auto with algebra.
apply Trans with (monoid_unit R); auto with algebra.
apply
Trans with (sgroup_law R (ring_mult x z) (ring_mult (group_inverse R y) z));
auto with algebra.
apply
Trans with (sgroup_law R (ring_mult x z) (group_inverse R (ring_mult y z)));
auto with algebra.
apply
Trans with (sgroup_law R (ring_mult x z) (group_inverse R (ring_mult x z)));
auto with algebra.
Qed.

Lemma INTEGRAL_DOMAIN_mult_eq_l :
forall x y z : R,
Equal (ring_mult z x) (ring_mult z y) ->
Equal x y \/ Equal z (monoid_unit R).