code: func (fs *RootMappingFs) Open(name string) (afero.File, error) {
  	if fs.isRoot(name) {
  		return &rootMappingFile{name: name, fs: fs}, nil
  	}
  	realName := fs.realName(name)
  	f, err := fs.Fs.Open(realName)
  	if err != nil {
  		return nil, err
  	}
  	return &rootMappingFile{File: f, name: name, fs: fs}, nil
  }
docs: // Open opens the named file for reading.