code: func (u *unionFS) bind(mountPoint string, fs http.FileSystem) {
  	u.ns[mountPoint] = fs
  	u.root.entries = append(u.root.entries, &dirInfo{
  		name: mountPoint[1:],
  	})
  }
docs: // bind mounts fs at mountPoint.
  // mountPoint must be of form "/mydir". It must start with a '/', and contain a single directory name.