def register_fts4_bm25(self):
        "Register the ``rank_bm25(match_info)`` function used for calculating relevance with SQLite FTS4."
        self.register_function(rank_bm25, deterministic=True, replace=True)
Register the ``rank_bm25(match_info)`` function used for calculating relevance with SQLite FTS4.
sqlite_utils/db.py