code: func (u *unionFS) Open(path string) (http.File, error) {
  	// TODO: Maybe clean path?
  	if path == "/" {
  		return &dir{
  			dirInfo: u.root,
  		}, nil
  	}
  	for prefix, fs := range u.ns {
  		if path == prefix || strings.HasPrefix(path, prefix+"/") {
  			innerPath := path[len(prefix):]
  			if innerPath == "" {
  				innerPath = "/"
  			}
  			return fs.Open(innerPath)
  		}
  	}
  	return nil, &os.PathError{Op: "open", Path: path, Err: os.ErrNotExist}
  }
docs: // Open opens the named file.