simpl in |- *.
unfold multfraction_fun, eqfraction in |- *; simpl in |- *.
intros x y z; try assumption.
apply
Trans
with
(ring_mult
(ring_mult (num x)
(sgroup_law R (ring_mult (num y) (den z))
(ring_mult (num z) (den y))))
(ring_mult (den x) (ring_mult (den y) (ring_mult (den x) (den z))))).
auto with algebra.
apply
Trans
with
(ring_mult
(ring_mult
(ring_mult (num x)
(sgroup_law R (ring_mult (num y) (den z))
(ring_mult (num z) (den y)))) (den x))
(ring_mult (den y) (ring_mult (den x) (den z)))).
auto with algebra.
apply RING_comp.
apply
Trans
with
(ring_mult
(sgroup_law R (ring_mult (num x) (ring_mult (num y) (den z)))
(ring_mult (num x) (ring_mult (num z) (den y))))
(den x)).
auto with algebra.
apply
Trans
with
(sgroup_law R
(ring_mult (ring_mult (num x) (ring_mult (num y) (den z))) (den x))
(ring_mult (ring_mult (num x) (ring_mult (num z) (den y))) (den x))).
auto with algebra.
apply SGROUP_comp.
apply
Trans
with (ring_mult (ring_mult (ring_mult (num x) (num y)) (den z)) (den x)).
auto with algebra.
apply
Trans
with (ring_mult (ring_mult (num x) (num y)) (ring_mult (den z) (den x)));
auto with algebra.
apply
Trans
with (ring_mult (ring_mult (ring_mult (num x) (num z)) (den y)) (den x)).
auto with algebra.
apply
Trans
with (ring_mult (ring_mult (num x) (num z)) (ring_mult (den y) (den x)));
auto with algebra.
auto with algebra.
Qed.

Lemma multfraction_com :
forall x y : fraction_set,
Equal (multfraction_fun x y) (multfraction_fun y x).