// here's some sample code

        private static PANEL p;

        private static IEnumerable<ScheduleMethod> switchStateOfButtonUsingButtonState()
        {
            // first get old state... -1 means DON'T change state of the button
            double oldState =
                p.button_state(1, -1);

            // then switch the state, according to the return value
            if (oldState == 0)
            {
                // switch to on
                p.button_state(1, 1);
            }
            else
            {
                // switch to off
                p.button_state(1, 0);
            }

            yield break;
        }

        public static IEnumerable<ScheduleMethod> myMainMethod()
        {
            // use mouse !!
            EngVar.mouse_mode = 4;

            p =
                PANEL.pan_create(null, 1);
            p.SHOW = true;
            p.pos_x = 100;
            p.pos_y = 100;
            p.size_x = 500;
            p.size_y = 500;

            BMAP on =
                BMAP.bmap_createblack(200, 200, 32);
            on.bmap_fill(new Color(0, 255, 0), 50);
            BMAP off =
                BMAP.bmap_createblack(200, 200, 32);
            off.bmap_fill(new Color(0, 0, 255), 50);
            BMAP over =
                BMAP.bmap_createblack(200, 200, 32);
            over.bmap_fill(new Color(200, 200, 20), 50);

            p.pan_setbutton(
                0,
                2,
                50,
                50,
                on,
                off,
                over,
                over,
                null,
                null,
                null);

            EngVar.on_space +=
                switchStateOfButtonUsingButtonState;

            yield break;
        }

        static void Main(string[] args)
        {
            EngFun.engine_open(null, null);
            EngFun.level_load(null);

            Scheduler.StartScheduler(myMainMethod);
        }