input: Write a python function to combine two lists into one.
  assert combine_lists([1, 2, 3], [4, 5, 6]) == [1, 2, 3, 4, 5, 6]
  assert combine_lists([], [7, 8, 9]) == [7, 8, 9]
  assert combine_lists(['a', 'b'], ['c', 'd']) == ['a', 'b', 'c', 'd']
output: def combine_lists(list1, list2):
      return list1 + list2