doIterate(aa,bb, mode); // a problem here is from flip or doIterate
fromIndex = random().nextInt(sz/2);
toIndex = fromIndex + random().nextInt(sz - fromIndex);
aa = (BitSet)a.clone(); aa.clear(fromIndex,toIndex);
bb = b.clone(); bb.clear(fromIndex,toIndex);
doNextSetBit(aa,bb); // a problem here is from clear() or nextSetBit
doPrevSetBit(aa,bb);