intuition.
unfold ratMult, natToPosnat, posnatMult.
eapply eqRat_refl.
Qed.

(* Specifications: *)
Theorem ratAdd_den_same : forall n1 n2 d,
RatIntro (n1 + n2)%nat d == (RatIntro n1 d) + (RatIntro n2 d).