code: func (b *Bitmap) writeOp(op *op) error {
  	if b.OpWriter == nil {
  		return nil
  	}
  
  	if _, err := op.WriteTo(b.OpWriter); err != nil {
  		return err
  	}
  	b.opN += op.count()
  
  	return nil
  }
docs: // writeOp writes op to the OpWriter, if available.