input: Write a function to generate a list of even numbers from 1 to 20.
  assert even_numbers() == [2, 4, 6, 8, 10, 12, 14, 16, 18, 20]
  assert even_numbers() != [1, 3, 5, 7, 9, 11, 13, 15, 17, 19]
output: def even_numbers():
      return [x for x in range(1, 21) if x % 2 == 0]