How to select valid lines for secondary list?

To stop the user from selecting invalid lines, ABAP/4 offers various possibilities.  At the end of the processing block END-OF-SELECTION, remove the contents of one or more fields you previously stored for valid lines using the HIDE statement.  At the event AT LINE-SELECTION, check whether the work area is initial or whether the HIDE statement stored field contents there.  After processing the secondary list, clear the work area again.  This prevents the user from trying to make further secondary lists from the secondary list displayed.