LET GUI_Widget

        GUI (Graphic-User-Interface) input mode:

    If  command  GUI\PANEL  has  been issued before, the LET command defines
    widgets in the master window defined by GUI\PANEL. The widget is a  sim-
    ple  prompt  when  option  /PROMPT  is set, a slider if option /RANGE is
    present, a list of choices if option /CHOICE or /INDEX is given,  and  a
    selection of files with the specified filter when option /FILE is speci-
    fied. If neither of these options is present, the LET command  works  in
    the usual way.

    The  widgets are created and activated by command GUI\GO. Standard input
    can be used, and pressing button GO will define  all  the  variables  as
    specified. If button ABORT is pressed instead, none of the variables are
    modified and an error is returned. Error handling is available.

    Options related to the GUI Widget mode are: /CHOICE,   /FILE,  /FORMULA,
    /INDEX, /PROMPT and /RANGE