public static function Rxp(array $r, array $p, array &$rp) {
    $w;
    $wrp = [];
    $i;
    $j;

    /* Matrix r * vector p. */
    for ($j = 0; $j < 3; $j++) {
      $w = 0.0;
      for ($i = 0; $i < 3; $i++) {
        $w += $r[$j][$i] * $p[$i];
      }
      $wrp[$j] = $w;
    }

    /* Return the result. */
    IAU::Cp($wrp, $rp);

    return;
  }