public static function Trxpv(array $r, array $pv, array &$trpv) {
    $tr = [];

    /* Transpose of matrix r. */
    IAU::Tr($r, $tr);

    /* Matrix tr * vector pv -> vector trpv. */
    IAU::Rxpv($tr, $pv, $trpv);

    return;
  }