input: Write a python function to generate a list of cubes of even numbers from 2 to 20.
  assert cube_even_numbers() == [8, 64, 216, 512, 1000, 1728, 2744, 4096, 5832, 8000]
output: def cube_even_numbers():
      return [x**3 for x in range(2, 21) if x % 2 == 0]