docs: public int compareTo(Piece other) {
      // return true for the same pieces, otherwise sort by time seen, then by index;
      if (this.equals(other)) {
        return 0;
      } else if (this.seen == other.seen) {
        return new Integer(this.index).compareTo(other.index);
      } else if (this.seen < other.seen) {
        return -1;
      } else {
        return 1;
      }
    }