b_cylin
if (ap->av_forw->b_cylin < ap->b_cylin ||
bp->b_cylin < ap->av_forw->b_cylin)
if (bp->b_cylin < ap->b_cylin) {
if (ap->av_forw->b_cylin < ap->b_cylin) {
if (bp->b_cylin < ap->av_forw->b_cylin)