def get_stable_id(self):
        """
        Return a stable id.

        :rtype: string
        """
        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}"
        )