public static boolean
isFlag(int index) {
	flags.check(index);
	if ((index >= 1 && index <= 4) || (index >= 12))
		return false;
	return true;
}
Indicates if a bit in the flags field is a flag or not.  If it's part of
the rcode or opcode, it's not.
src/main/java/core/org/xbill/DNS/Flags.java