# any unexpected bits mean failure, and then known set bits are ignored
# if no unexpected bits set, operation continues (subject to meaning
# of any expected bits set). So, eg, exit 0 means "continue normally"
# any unexpected bits mean failure, and then known set bits are ignored
# if no unexpected bits set, operation continues (subject to meaning
# of any expected bits set). So, eg, exit 0 means "continue normally"