public static function xyzToFace(S2Point $p)
 {
     $face = $p->largestAbsComponent();
     if ($p->get($face) < 0) {
         $face += 3;
     }
     return $face;
 }
 public static function longitude(S2Point $p)
 {
     // Note that atan2(0, 0) is defined to be zero.
     return S1Angle::sradians(atan2($p->get(1), $p->get(0)));
 }