code: func (d *dir) mkdir(name string) (*dir, error) {
  	if name == "" {
  		return nil, fmt.Errorf("invalid dirname")
  	}
  
  	_, ok := d.children[name]
  	if ok {
  		return nil, errExists
  	}
  
  	child := &dir{
  		common: common{
  			p:   path.Join(d.path(), name),
  			mod: time.Now(),
  		},
  	}
  	d.add(child)
  	d.mod = time.Now()
  
  	return child, nil
  }
docs: // mkdir creates a child directory under d with the given name.