code: func New(mapping map[string]http.FileSystem) http.FileSystem {
  	u := &unionFS{
  		ns: make(map[string]http.FileSystem),
  		root: &dirInfo{
  			name: "/",
  		},
  	}
  	for mountPoint, fs := range mapping {
  		u.bind(mountPoint, fs)
  	}
  	return u
  }
docs: // New creates an union filesystem with the provided mapping of mount points to filesystems.
  //
  // Each mount point must be of form "/mydir". It must start with a '/', and contain a single directory name.