input: Write a function to implement the add method in the Sum class so that it returns the sum of the two integers x and y. If one or both of the integers are missing, the method should return None.
  
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
      
      def add(self):
          if self.x is None or self.y is None:
              return None
          return self.x + self.y
  
  assert Sum(3, 5).add() == 8
  assert Sum(0, 0).add() == 0
  assert Sum(-3, 3).add() == 0
  assert Sum(None, 5).add() == None
  assert Sum(3, None).add() == None
  assert Sum(None, None).add() == None
output: def add(self):
      if self.x is None or self.y is None:
          return None
      return self.x + self.y