Before you use the animation system, you must have called the routine
initgels().  the section called "bob priorities" describes how the system
maintains the list of GELs to draw on the screen according to their
various data fields.  The animation system selectively adds GELs to and
removes GELs from this list of screen objects during the animate()
routine.  On the next call to drawglist(), the system will draw the gels
in the list into the selected rastport.