Description

   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.