Definition Qabs (q:Q):Q:= 
  match q with 
  | Zero => Zero
  | Qpos qp => Qpos qp
  | Qneg qp => Qpos qp
  end.

Lemma Qabs_Qmult: forall q1 q2, Qabs (q1*q2) = (Qabs q1) * (Qabs q2).