This routine controls the editing session. It is responsible for
    reading the text and commands and for executing them. Windows on
    the screen are updated to reflect the edits made. Your program
    can regain control by interrupting TPU using the TPU$SPECIFY_
    ASYNC_ACTION routine, together with the TPU$TRIGGER_ASYNC_ACTION
    routine.

                                   NOTE

       Control is also returned to your program if an error occurs
       or when you enter either the built-in procedure QUIT or the
       built-in procedure EXIT.