From: Leopold Toetsch <lt@[...].at> 
   Date: Sat, 16 Jul 2005 11:38:41 +0200

   . . .

   We keep the invariant by several means:
   . . .
   c) a write barrier checks pointer stores into aggregates (by just 
   comparing 2 memory addresses - basically)
      we can do either:
      - make aggregate younger (move to left)
      - make stored-into object older (move to right)
      - remember in IGP

What happens when a store creates a cycle?  And how would this be
detected?

					-- Bob