Usually if a window invokes a call-back function (in response to a menu, say)
most of the standard controls are inhibited from raising further call-backs.
This action is normally desirable as it prevents unfortunate complications if
the user's mouse 'bounces' on a button.
The subroutine PERMIT_ANOTHER_CALLBACK@ allows a call-back function to permit an
additional call-back. This routine will only permit one additional call-back,
but it can be called as many times as desired, each time enabling a further
call-back. When using this routine it is assumed that a call will also be made
to a routine such as TEMPORARY_YIELD@ to permit the processing of further