code: func Open(name string) (*FS, error) {
  	var off int64
  	if i := strings.Index(name, "@"); i >= 0 {
  		v, err := strconv.ParseInt(name[i+1:], 0, 64)
  		if err != nil {
  			return nil, fmt.Errorf("invalid offset in name %q", name)
  		}
  		off = v
  		name = name[:i]
  	}
  
  	f, err := os.Open(name)
  	if err != nil {
  		return nil, err
  	}
  
  	var r io.ReaderAt = f
  	if off != 0 {
  		r = &readerAtOffset{r, off}
  	}
  
  	fs, err := Init(r)
  	if err != nil {
  		f.Close()
  		return nil, err
  	}
  	fs.c = f
  	return fs, nil
  }
docs: // Open opens the file system in the named file.
  //
  // If the name contains an @ sign, it is taken to be
  // of the form file@offset, where offset is a decimal,
  // hexadecimal, or octal number according to its prefix,
  // and the file system is assumed to start at the given
  // offset in the file instead of at the beginning of the file.