new_contents: from ch02.and_gate02 import AND
  from ch02.nand_gate import NAND
  from ch02.or_gate import OR
  
  
  def XOR(x1, x2):
      s1 = NAND(x1, x2)
      s2 = OR(x1, x2)
      return AND(s1, s2)
  
  
  if __name__ == '__main__':
      for xs in [(0, 0), (1, 0), (0, 1), (1, 1)]:
          y = XOR(xs[0], xs[1])
          print(str(xs) + " -> " + str(y))
  
subject: Add xor_gate