public static function Rxr(array $a, array $b, array &$atb) {
    $i;
    $j;
    $k;
    $w;
    $wm = [];

    for ($i = 0; $i < 3; $i++) {
      for ($j = 0; $j < 3; $j++) {
        $w = 0.0;
        for ($k = 0; $k < 3; $k++) {
          $w += $a[$i][$k] * $b[$k][$j];
        }
        $wm[$i][$j] = $w;
      }
    }
    IAU::Cr($wm, $atb);

    return;
  }