intuition.
unfold ratAdd, ratMult.
rattac.
arithNormalize.
arithSimplify.
do 4 arithSimplify.
Qed.
Definition numerator r :=
match r with
| RatIntro n d => n
end.

(* Specifications: *)
Lemma ratDistance_add_same_l_gen : forall r1 r2 r3 r4,
r1 == r3 ->
ratDistance (r1 + r2) (r3 + r4) == ratDistance r2 r4.