
theorem
  43 is prime
proof
  now
    43 = 2*21 + 1; hence not 2 divides 43 by NAT_4:9;
    43 = 3*14 + 1; hence not 3 divides 43 by NAT_4:9;
    43 = 5*8 + 3; hence not 5 divides 43 by NAT_4:9;
  end;
  then for n being Element of NAT st 1 < n & n*n <= 43 & n is prime
  holds not n divides 43 by XPRIMET1:6;
  hence thesis by NAT_4:14;
