docs: def _cached(self, dbname, coll, index):
          """Test if `index` is cached."""
          cache = self.__index_cache
          now = datetime.datetime.utcnow()
          with self.__index_cache_lock:
              return (dbname in cache and
                      coll in cache[dbname] and
                      index in cache[dbname][coll] and
                      now < cache[dbname][coll][index])