class Interval:
   """
   This class illustrates basic interval arithmetic.
   """
   def __init__(self,a,*b):
      "returns the interval (a,b)"
      self.a = float(a)
      if len(b) == 0:
         self.b = float(a)
      else:
         self.b = float(b[0])

   def __str__(self):
      "return string representation"
      s = '[%f,%f]' % (self.a,self.b)
      return s

   def __repr__(self):
      "calls the string representation"
      return str(self)

   def __add__(self,other):
      "returns addition of two intervals"
      x = self.a + other.a
      y = self.b + other.b
      return Interval(x,y)

   def __sub__(self,other):
      "return subtraction of two intervals"
      x = self.a - other.b
      y = self.b - other.a
      return Interval(x,y)

   def __mul__(self,other):
      "returns multiplication of two intervals"
      ac = self.a*other.a
      ad = self.a*other.b
      bc = self.b*other.a
      bd = self.b*other.b
      L = [ac,ad,bc,bd]
      return Interval(min(L),max(L))

   def __div__(self,other):
      "returns division of two intervals"
      ac = self.a/other.a
      ad = self.a/other.b
      bc = self.b/other.a
      bd = self.b/other.b
      L = [ac,ad,bc,bd]
      return Interval(min(L),max(L))