selection screen 화면에서

    CALL FUNCTION 'REUSE_ALV_GRID_DISPLAY'
      EXPORTING
        i_background_id          = 'ALV_BACKGROUND'
        i_callback_program       = sy-repid
        i_callback_user_command  = 'USER_COMMAND'
        i_callback_pf_status_set = 'SET_PF_STATUS'
        it_fieldcat              = gt_fieldcat
        it_sort                  = gt_sortcat
        i_save                   = 'A'         "A or U
        is_layout                = gt_layout
        it_events                = gt_events[]
      IMPORTING
        es_exit_caused_by_user   = ls_exit_by_user
      TABLES
        t_outtab                 = gt_result_alv
      EXCEPTIONS
        program_error            = 1
        OTHERS                   = 2.

이용해서 화면에 리스트를 뿌려주고 상단의 기능키 동작을 아래처럼 하고싶은데요

    CHECK ls_exit_by_user-back = 'X'.
    CLEAR ls_exit_by_user.
    LEAVE TO SCREEN 0.

    CHECK ls_exit_by_user-exit = 'X'.
    CLEAR ls_exit_by_user.
    LEAVE TO SCREEN 0.

    CHECK ls_exit_by_user-cancel = 'X'.
    CLEAR ls_exit_by_user.
    LEAVE PROGRAM.

back 는 되는데.. 나머지  exit 이나, cancel 은 반응이 없습니다.

어떻게 해야 하나요?