code: func (f *Files) Add(filename string, size int64, modtime time.Time, o Opener) {
  	f.lk.Lock()
  	defer f.lk.Unlock()
  
  	r, err := o.Open()
  	if err != nil {
  		log.Printf("Could not add file %v: %v", filename, err)
  		return
  	}
  	contents, err := ioutil.ReadAll(r)
  	if err != nil {
  		log.Printf("Could not read contents of file %v: %v", filename, err)
  		return
  	}
  
  	f.add(filename, &staticFile{
  		name:     filename,
  		contents: contents,
  		modtime:  modtime,
  	})
  }
docs: // Add adds a file to the file set.