input: 
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y
          
  # Testing the Sum class
  # Example 1: Sum(3, 5).add() should return 8.
  # Example 2: Sum(-1, 1).add() should return 0.
  
output:     
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y
          
  # Testing the Sum class
  # Example 1: Sum(3, 5).add() should return 8.
  # Example 2: Sum(-1, 1).add() should return 0.
  
  # Example 1
  assert Sum(3, 5).add() == 8
  
  # Example 2
  assert Sum(-1, 1).add() == 0
  