code: func (f *fragment) incrementOpN() error {
  	f.opN++
  	if f.opN <= f.MaxOpN {
  		return nil
  	}
  
  	if err := f.snapshot(); err != nil {
  		return fmt.Errorf("snapshot: %s", err)
  	}
  	return nil
  }
docs: // incrementOpN increase the operation count by one.
  // If the count exceeds the maximum allowed then a snapshot is performed.