input: 
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y
  """
  >>> s = Sum(3, 5)
  >>> s.add()
  8
  >>> s = Sum(-1, 1)
  >>> s.add()
  0
  """
  
output:     
      def __str__(self):
          return f"Sum({self.x}, {self.y})"
  