@Override
  public String toString() {
    StringBuilder b = new StringBuilder();
    b.append('{');

    for (int i = nextSetBit(0); i >= 0; i = nextSetBit(i + 1)) {
      int endOfRun = nextClearBit(i);
      if (endOfRun - i > 1) {
        if (b.length() != 1) {
          b.append(", ");
        }
        b.append(i).append("...").append(endOfRun-1);
        i = endOfRun;
      }
      else {
        do {
          if (b.length() != 1) {
            b.append(", ");
          }
          b.append(i);
        }
        while (++i < endOfRun);
      }
    }

    b.append('}');
    return b.toString();
  }
@Override
  public String toString()