
theorem
  23 is prime
proof
  now
    23 = 2*11 + 1; hence not 2 divides 23 by NAT_4:9;
    23 = 3*7 + 2; hence not 3 divides 23 by NAT_4:9;
  end;
  then for n being Element of NAT st 1 < n & n*n <= 23 & n is prime
  holds not n divides 23 by XPRIMET1:4;
  hence thesis by NAT_4:14;
end;
