code: def get_stable_id(self):
          
          doc_id, _, parent_doc_char_start, _ = split_stable_id(self.sentence.stable_id)
          return (
              f"{self.sentence.document.name}"
              f"::"
              f"{self._get_polymorphic_identity()}"
              f":"
              f"{parent_doc_char_start + self.char_start}"
              f":"
              f"{parent_doc_char_start + self.char_end}"
              f":"
              f"{self.expander_key}"
              f":"
              f"{self.position}"
          )
docs: Return a stable id.
  
          :rtype: string