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