code: func (f Index) Has(keyFields Item) (bool, error) {
  	key, err := f.encodeKeyFunc(keyFields)
  	if err != nil {
  		return false, err
  	}
  	return f.db.Has(key)
  }
docs: // Has accepts key fields represented as Item to check
  // if there this Item's encoded key is stored in
  // the index.