input: 
  from typing import Type
  
  def add_method_to_sum_class(SumClass: Type[Sum]) -> None:
      """ Adds a method to the Sum class that calculates the sum of two integers.
      >>> sum_instance = Sum(3, 5)
      >>> sum_instance.add()
      8
      >>> sum_instance_2 = Sum(-1, 1)
      >>> sum_instance_2.add()
      0
      """
      def add(self):
          return self.x + self.y
      
      setattr(SumClass, 'add', add)
  
output:     def add(self):
          return self.x + self.y
      
      setattr(SumClass, 'add', add)