input: Write a function to double each element in a list and print the results.
  assert double_elements([1, 2, 3, 4, 5]) == [2, 4, 6, 8, 10]
  assert double_elements([10, 20, 30]) == [20, 40, 60]
  assert double_elements([7, 14, 21]) == [14, 28, 42]
output: def double_elements(lst):
      return [x * 2 for x in lst]