_imarks
(l)->_imarks.m1 = (l)->_imarks.m2 = (l)->_imarks.m3 = \
(l)->_imarks.m4 = (l)->_imarks.m5 = (l)->_imarks.m6 = \
(l)->_imarks.m7 = (l)->_imarks.m8 = EMPTY_SET)