BLACKBIT
#define maskcolors (~(bit2mask(BLACKBIT, OLDBIT) | WHITEBITS))
#define black2gray(x) resetbit(gch(x)->marked, BLACKBIT)
#define isblack(x) testbit((x)->gch.marked, BLACKBIT)
(!testbits((x)->gch.marked, WHITEBITS | bitmask(BLACKBIT)))
#define gray2black(x) l_setbit((x)->gch.marked, BLACKBIT)