code: func (f *fileInZip) prefetch() error {
  	if f.body != nil {
  		return nil
  	}
  	r, err := f.z.Open()
  	if err != nil {
  		return err
  	}
  	defer r.Close()
  	f.body, err = ioutil.ReadAll(r)
  	return err
  }
docs: // prefetch loads the body of file into memory to speed up later calls.