input: Write a python function to expand a list by doubling its elements without modifying the original list during iteration.
  assert expand_list([1, 2, 3]) == [1, 2, 3, 1, 2, 3]
  assert expand_list(['a', 'b']) == ['a', 'b', 'a', 'b']
  assert expand_list([]) == []
output: def expand_list(lst):
      return lst + lst