bool operator <(Quadruple<t> const &qplIn) const
{
    return (tX < qplIn.tX
            || tX == qplIn.tX
               && (tY < qplIn.tY
                   || tY == qplIn.tY
                      && (tZ < qplIn.tZ
                          || tZ == qplIn.tZ
                             && tW < qplIn.tW)));
}