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 Zmult_absorb : forall x y z : Z, x <> 0%Z -> (x * y)%Z = (x * z)%Z -> y = z.