public static function categories()
 {
     if (empty(self::$categories)) {
         self::$categories = array("A" => new ABC_Intervall_impl("A", 0.0, 75.0), "B" => new ABC_Intervall_impl("B", 75.0, 95.0), "C" => new ABC_Intervall_impl("C", 95.0, 101.0));
     }
     return self::$categories;
 }