Definition projP1 (H : sig P) := let (x, h) := H in x.Definition projP2 (H : sig P) :=
   let (x, h) as H return (P (projP1 H)) := H in h.

Lemma Zsimpl_mult_l : forall n m p : Z, n <> 0%Z -> (n * m)%Z = (n * p)%Z -> m = p.