input: Write a python function to append one list to another.
  assert append_list([1, 2, 3], [4, 5, 6]) == [1, 2, 3, 4, 5, 6]
  assert append_list([], [1, 2, 3]) == [1, 2, 3]
  assert append_list([1, 2, 3], []) == [1, 2, 3]
  assert append_list([], []) == []
  assert append_list(['a', 'b'], ['c', 'd']) == ['a', 'b', 'c', 'd']
output: def append_list(list1, list2):
      return list1 + list2