code: func (w *Writer) Add(h plumbing.Hash, pos uint64, crc uint32) {
  	w.m.Lock()
  	defer w.m.Unlock()
  
  	if w.added == nil {
  		w.added = make(map[plumbing.Hash]struct{})
  	}
  
  	if _, ok := w.added[h]; !ok {
  		w.added[h] = struct{}{}
  		w.objects = append(w.objects, Entry{h, crc, pos})
  	}
  
  }
docs: // Add appends new object data.