code: def expand(self):
          
          old_expanded = [[]]
          for sub in self._tree:
              sub_expanded = sub.expand()
              new_expanded = []
              while len(old_expanded) > 0:
                  sentence = old_expanded.pop()
                  for new in sub_expanded:
                      new_expanded.append(sentence + new)
              old_expanded = new_expanded
          return old_expanded
docs: Creates a combination of all sub-sentences.
  
          Returns:
              List<List<str>>: A list with all subsentence expansions combined in
                                  every possible way