rattac.
inversion H1; clear H1; subst.
rewrite mult_1_r.
rewrite mult_1_l.
trivial.
Qed.

(* Specifications: *)
Lemma rat_remove_common_factor : forall (n num : nat)(nzn : nz n) den,
RatIntro (n * num) (posnatMult (natToPosnat nzn) den) == RatIntro num den.