input: 
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y
  
  """
  >>> s = Sum(2, 3)
  >>> s.add()
  5
  >>> s = Sum(-1, 1)
  >>> s.add()
  0
  """
  
output:     
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y
  