static Bits joinBits(Collection<Bits> l) {
  if (empty(l)) ret new Bits;
  new Bits bits;
  for (Bits b : l)
    appendBits(bits, b);
  ret bits;
}

static Bits joinBits(Bits a, Bits b) {
  if (a == null) ret b;
  if (b == null) ret a;
  int n = a.n;
  BitSet bs = b.bs;
  Bits c = new Bits(cloneBitSet(a.bs), n+b.n);
  BitSet bs2 = c.bs;
  for (int i = bs.nextSetBit(0); i >= 0; i = bs.nextSetBit(i+1))
    bs2.set(n+i);
  ret c;
}