input: Write a function to merge two lists.
  assert merge_two_lists([1, 2, 3], [4, 5, 6]) == [1, 2, 3, 4, 5, 6]
  assert merge_two_lists([], [1, 2, 3]) == [1, 2, 3]
  assert merge_two_lists(['a', 'b'], ['c', 'd']) == ['a', 'b', 'c', 'd']
output: def merge_two_lists(list1, list2):
      return list1 + list2