input: Write a function to fix the provided Python function that attempts to modify a list while iterating over it. Ensure that the list is copied before the iteration starts to avoid unexpected behavior. The function should take a list as an input and return the modified list based on the expansion operation described.
  assert expand_list([1, 2, 3])==[1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3]
  assert expand_list([])==[]
  assert expand_list([5])==[5, 5, 5, 5]
  assert expand_list([1, 2])==[1, 2, 1, 2, 1, 2, 1, 2]
output: def expand_list(lst):
      copied_lst = lst[:]
      for _ in range(5):
          lst.extend(copied_lst)
      return lst