input: Write a python function to double the elements of a list using the given double function.
  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([0, -1, -2]) == [0, -2, -4]
output: def double_elements(lst):
      def double(x):
          return x * 2
      return [double(x) for x in lst]