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