
theorem
  for n being Nat holds n = PN-to-NAT.(NAT-to-PN.n)
proof
  defpred P[Nat] means $1 = PN-to-NAT.(NAT-to-PN.$1);
A1: P[0] by Lm17;
A2: for n being Nat st P[n] holds P[n+1] by Lm18;
  thus for n being Nat holds P[n] from NAT_1:sch 2(A1,A2);
end;
