text: from util_pair import cons, car, cdr
  from util_interval import make_interval, print_interval
  from util_interval import add_interval, mul_interval, div_interval
  
  def lower_bound(z):
      return min([car(z), cdr(z)])
  
  def upper_bound(z):
      return max([car(z), cdr(z)])
  
  z1 = make_interval(2, 5)
  z2 = make_interval(7, 3)
  
  print_interval(z1)
  print_interval(z2)
  
  print_interval(add_interval(z1, z2))
  print_interval(mul_interval(z1, z2))
  print_interval(div_interval(z1, z2))