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;

}