static void appendBits(Bits a, Bits b) {
  int n = a.n;
  BitSet bs = b.bs, bs2 = a.bs;
  for (int i = bs.nextSetBit(0); i >= 0; i = bs.nextSetBit(i+1))
    bs2.set(n+i);
  a.n += b.n;
}