Problem with sub-screen <actions on Cancel button>

Hi all,

I have a problem with sub-screen.

I created a subscreen (screen sequence in MM01/MM02/MM03). When users leave this screen, I call POPUP_TO_CONFIRM to ask if they want to save the data.

1) My goal is: when they choose Cancel, no actions are performed and users will see the current screen. However, I cannot use command: SET SCREEN 0 or LEAVE TO SCREEN 0 because the system gives an error message:

 *SET SCREEN not allowed in subscreens*. 

2) How can we determine when the user leaves the screen? which value of SY-UCOMM will be used to check this?

Thanks in advance.