doIterate(aa,bb); // a problem here is from flip or doIterate
fromIndex = rand.nextInt(sz+80);
toIndex = fromIndex + rand.nextInt((sz>>1)+1);
aa = (BitSet)a.clone(); aa.clear(fromIndex,toIndex);
bb = (OpenBitSet)b.clone(); bb.clear(fromIndex,toIndex);
doNextSetBit(aa,bb); // a problem here is from clear() or nextSetBit
fromIndex = rand.nextInt(sz+80);