TODO
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NOTE:  nested enums are broken do to dynamic binning.

Don't rebuild entire bplane when bbox grows.
unsubbing sometimes.


Some (remaining) bplane design issues:

groups?
	idea: support small number of groups by having
	separate bplane for each group.  Do selection this
	way.  Logically layered on top of bplanes - but may	
	want to integrate.  
	DECISION:  defer for now.

pack/unpack?
	Seems incompatible with user alloc/dealloc.
	Complicated.
	DECISION:  Forget it.

coarse/fine with cache.
	can be added later.  should fit in nicely.
	no special support needed.

integrated find/add?
	can add later.  
	don't worry about it now.
	
May want to add attributes, via external hash.