intros x y y' H'; try assumption.
apply Trans with (addfraction_fun y x).
apply addfraction_fun_com.
apply Trans with (addfraction_fun y' x).
apply addfraction_law_l; auto with algebra.
apply addfraction_fun_com.
Qed.

(* Specifications: *)
Lemma addfraction_law : fun2_compatible addfraction_fun.
red in |- *.