code: func RetrieveChecksum(in IndexInput) (int64, error) {
  	var err error
  	if err = in.Seek(in.Length() - FOOTER_LENGTH); err != nil {
  		return 0, err
  	}
  	if err = validateFooter(in); err != nil {
  		return 0, err
  	}
  	return in.ReadLong()
  }
docs: /* Returns (but does not validate) the checksum previously written by CheckFooter. */