
theorem
  79 is prime
proof
  now
    79 = 2*39 + 1; hence not 2 divides 79 by NAT_4:9;
    79 = 3*26 + 1; hence not 3 divides 79 by NAT_4:9;
    79 = 5*15 + 4; hence not 5 divides 79 by NAT_4:9;
    79 = 7*11 + 2; hence not 7 divides 79 by NAT_4:9;
  end;
  then for n being Element of NAT st 1 < n & n*n <= 79 & n is prime
  holds not n divides 79 by XPRIMET1:8;
  hence thesis by NAT_4:14;
