input: Write a function to generate even numbers using a generator function.
  assert [next(even_number_generator()) for _ in range(5)] == [0, 2, 4, 6, 8]
  assert [next(even_number_generator()) for _ in range(3)] == [0, 2, 4]
  assert [next(even_number_generator()) for _ in range(7)] == [0, 2, 4, 6, 8, 10, 12]
output: def even_number_generator():
      n = 0
      while True:
          yield n
          n += 2