intros x; try assumption.
case x; unfold eqfraction, fraction0 in |- *; simpl in |- *.
intros numer denom H' H'0; try assumption.
apply Trans with (ring_mult numer (ring_unit R)); auto with algebra.
apply Trans with (ring_mult (monoid_unit R) denom); auto with algebra.
Qed.

Lemma eqfraction_num0 :
forall x : fraction, Equal (num x) (monoid_unit R) -> eqfraction x fraction0.