input:                         
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          """ Calculates the sum of two integers.
          
          >>> Sum(3, 4).add()
          7
          >>> Sum(-1, 1).add()
          0
          """
          return self.x + self.y
output:     return self.x + self.y