public static function C2ixys($x, $y, $s, array &$rc2i) {
    $r2;
    $e;
    $d;

    /* Obtain the spherical angles E and d. */
    $r2 = $x * $x + $y * $y;
    $e  = ($r2 > 0.0) ? atan2($y, $x) : 0.0;
    $d  = atan(sqrt($r2 / (1.0 - $r2)));

    /* Form the matrix. */
    IAU::Ir($rc2i);
    IAU::Rz($e, $rc2i);
    IAU::Ry($d, $rc2i);
    IAU::Rz(-($e + $s), $rc2i);

    return;
  }