def arc_description(self, start: TLineNo, end: TLineNo) -> str:
        """Provide an English description of an arc's effect."""
        if self._missing_arc_fragments is None:
            self._analyze_ast()
            assert self._missing_arc_fragments is not None

        fragment_pairs = self._missing_arc_fragments.get((start, end), [(None, None)])
        action_msg = self._finish_action_msg(fragment_pairs[0][1], end)
        return action_msg
Provide an English description of an arc's effect.
coverage/parser.py