intros a b c H H0 H1; rewrite H1; rewrite Qpositive_plus_sym;
rewrite Qpositive_sub_correct2; auto with *.
Qed.

(* Specifications: *)
Theorem Qplus_assoc_two_neg :
forall (n : Q) (m' p' : Qpositive),
Qplus n (Qplus (Qneg m') (Qneg p')) = Qplus (Qplus n (Qneg m')) (Qneg p').