public static function get_inference_result(Language_variable_collection $langvar_coll, $method = 'closure_addition')
 {
     if (is_null(Fuzzy_inference_engine::$engine)) {
         $class_name = 'Fuzzy_inference_engine_' . $method;
         require_once $class_name . '.php';
         Fuzzy_inference_engine::$engine = new $class_name();
     }
     return Fuzzy_inference_engine::$engine->get_inference_result($langvar_coll);
 }