unfold addfraction_fun in |- *; simpl in |- *.
unfold eqfraction in |- *; simpl in |- *.
intros x y; try assumption.
apply RING_comp; auto with algebra.
Qed.

Lemma addfraction_law_r :
forall x y y' : fraction_set,
Equal y y' -> Equal (addfraction_fun x y) (addfraction_fun x y').