This subsection describes the choices for inter-Bob priorities.  The
inter-Bob priorities tell the system what order to render the Bobs.  Bobs
rendered earlier will appear to be behind later Bobs.  A Bob drawn earlier
is said to have the lower priority and a Bob drawn later is said to have
the higher priority.  Thus, the highest priority Bob will be drawn last
and will never be obstructed by another Bob.

 letting the system decide priorities 
 specifying the drawing order