Definition pred_nat : forall (x : Z) (Hx : (0 < x)%Z), nat.
intros [| px| px] Hx; try abstract (discriminate Hx).
exact (pred (nat_of_P px)).
Defined.

Lemma pred_nat_equal :
forall (x : Z) (Hx1 Hx2 : (0 < x)%Z), pred_nat x Hx1 = pred_nat x Hx2.