public static function Eors(array $rnpb, $s) {
    $x;
    $ax;
    $xs;
    $ys;
    $zs;
    $p;
    $q;
    $eo;

    /* Evaluate Wallace & Capitaine (2006) expression (16). */
    $x  = $rnpb[2][0];
    $ax = $x / (1.0 + $rnpb[2][2]);
    $xs = 1.0 - $ax * $x;
    $ys = -$ax * $rnpb[2][1];
    $zs = -$x;
    $p  = $rnpb[0][0] * $xs + $rnpb[0][1] * $ys + $rnpb[0][2] * $zs;
    $q  = $rnpb[1][0] * $xs + $rnpb[1][1] * $ys + $rnpb[1][2] * $zs;
    $eo = (($p != 0) || ($q != 0)) ? $s - atan2($q, $p) : $s;

    return $eo;
  }