input: Write a function that demonstrates the functionality of the Sum class by adding two integers and running assertions to ensure the method is correct. The function should return the result of the sum of two integers using the add method of the Sum class.
  assert Sum(2, 3).add() == 5
  assert Sum(0, 0).add() == 0
  assert Sum(-1, 1).add() == 0
  assert Sum(-3, -7).add() == -10
output: def test_sum():
      return Sum(2, 3).add()