Helper to clear the docs on RESET or filter mismatch.

def _reset_docs(self):
        """
        Helper to clear the docs on RESET or filter mismatch.
        """
        _LOGGER.debug("resetting documents")
        self.change_map.clear()
        self.resume_token = None

        # Mark each document as deleted. If documents are not deleted
        # they will be sent again by the server.
        for snapshot in self.doc_tree.keys():
            name = snapshot.reference._document_path
            self.change_map[name] = ChangeType.REMOVED

        self.current = False