End ZpRing.
Definition Zp_trunc p := p.-2.
Notation "''Z_' p" := 'I_(Zp_trunc p).+2
(at level 8, p at level 2, format "''Z_' p") : type_scope.
Notation "''F_' p" := 'Z_(pdiv p)
(at level 8, p at level 2, format "''F_' p") : type_scope.
Arguments natr_Zp {p'} x.
Section Groups.
Variable p : nat.
Definition Zp := if p > 1 then [set: 'Z_p] else 1%g.
Definition units_Zp := [set: {unit 'Z_p}].

Lemma Zp_cast : p > 1 -> (Zp_trunc p).+2 = p.