 {  iter := cfg.BorrowIterator(data)  defer cfg.ReturnIterator(iter)  iter.ReadVal(v)  c := iter.nextToken()  if c == 0 {   if iter.Error == io.EOF {    return nil   }   return iter.Error  }  iter.ReportError("Unmarshal", "there are bytes left after unmarshal")  return iter.Error }  func (cfg *frozenConfig) 