Description

   This routine is used with the built-in procedure CALL_USER and
   its associated call-user routine to control the TPU access
   to the terminal. When a call-user routine invokes TPU$CLOSE_
   TERMINAL, TPU closes its channel to the terminal and the
   channel of the TPU associated mailbox.

   When the call-user routine returns control to it, TPU
   automatically reopens a channel to the terminal and redisplays
   the visible windows.

   A call-user routine can use TPU$CLOSE_TERMINAL at any point in
   the program and as many times as necessary. If the terminal is
   already closed to TPU when TPU$CLOSE_TERMINAL is used, the
   call is ignored.