SEGSUM_IINFOSTART
iibase = SEGSUM_IINFOSTART(fs, sp);
iip = SEGSUM_IINFOSTART(fs, bp->b_data);
((IINFO *)((char *)SEGSUM_IINFOSTART(fs, buf) - (n)*IINFOSIZE(fs)))
iip = SEGSUM_IINFOSTART(fs, buf);
iip = SEGSUM_IINFOSTART(lfsp, sp);
iip2 = SEGSUM_IINFOSTART(lfsp, sp);