intuition.
repeat rewrite (ratMult_comm r3).
eapply ratMult_ratDistance_factor_r.
Qed.

(* Specifications: *)
Lemma ratAdd_rel_left_total : forall (r1 r2: Rat -> Prop),
(exists r1', r1 r1') ->
(exists r2', r2 r2') ->
(forall x1 x2, r1 x1 -> r1 x2 -> x1 == x2) ->
(forall x1 x2, r2 x1 -> r2 x2 -> x1 == x2) ->
exists r3, ratAdd_rel r1 r2 r3.