code: func (f Index) Put(i Item) (err error) {
  	key, err := f.encodeKeyFunc(i)
  	if err != nil {
  		return err
  	}
  	value, err := f.encodeValueFunc(i)
  	if err != nil {
  		return err
  	}
  	return f.db.Put(key, value)
  }
docs: // Put accepts Item to encode information from it
  // and save it to the database.