docs: def _add_to(self, db, index, item, default=OOSet):
          """
          Add `item` to `db` under `index`. If `index` is not yet in `db`, create
          it using `default`.
  
          Args:
              db (dict-obj): Dict-like object used to connect to database.
              index (str): Index used to look in `db`.
              item (obj): Persistent object, which may be stored in DB.
              default (func/obj): Reference to function/object, which will be
                  used to create the object under `index`.
                  Default :class:`OOSet`.
          """
          row = db.get(index, None)
  
          if row is None:
              row = default()
              db[index] = row
  
          row.add(item)