public static function get_ruleoutcome_list() {
        static $list = null;

        if ($list === null) {
            $list = array(
                self::OUTCOME_NONE => self::get_ruleoutcome_name(self::OUTCOME_NONE),
                self::OUTCOME_EVIDENCE => self::get_ruleoutcome_name(self::OUTCOME_EVIDENCE),
                self::OUTCOME_RECOMMEND => self::get_ruleoutcome_name(self::OUTCOME_RECOMMEND),
                self::OUTCOME_COMPLETE => self::get_ruleoutcome_name(self::OUTCOME_COMPLETE));
        }

        return $list;
    }