OLDBIT
#define maskcolors (~(bit2mask(BLACKBIT, OLDBIT) | WHITEBITS))
toset = bitmask(OLDBIT); /* set the old bit of all surviving objects */
tostop = bitmask(OLDBIT); /* do not sweep old generation */
#define isold(x) testbit((x)->gch.marked, OLDBIT)
#define resetoldbit(o) resetbit((o)->gch.marked, OLDBIT)