code: func expectedMemberSize(h gzip.Header) int {
  	i := bytes.Index(h.Extra, bgzfExtraPrefix)
  	if i < 0 || i+5 >= len(h.Extra) {
  		return -1
  	}
  	return (int(h.Extra[i+4]) | int(h.Extra[i+5])<<8) + 1
  }
docs: // expectedMemberSize returns the size of the BGZF conformant gzip member.
  // It returns -1 if no BGZF block size field is found.