input: Write a python function to double the elements of a list.
  assert double_elements([1, 2, 3]) == [2, 4, 6]
  assert double_elements([0, -1, -2]) == [0, -2, -4]
  assert double_elements([5, 10, 15]) == [10, 20, 30]
output: def double_elements(lst):
      return [x * 2 for x in lst]