ISSUE: BNode scope through UNION (and other algebraic operations)

This got a bit mixed in with the DISTINCT discussion, but it's worth  
breaking out since I don't immediately see where it's specced.

E.g., not in:
	http://www.w3.org/2001/sw/DataAccess/rq23/rq24.html#AltMatchingDefn

There are several "levels" of scoping possible. See:
	http://www.cs.man.ac.uk/~bparsia/2006/row-tutorial/#slide27

Through
	http://www.cs.man.ac.uk/~bparsia/2006/row-tutorial/#slide31

I suspect that the current "understanding" is query scoped BNodes.  
But I don't see where it's specified. (Pointer welcome.) It's worht  
considering table scoping. While less expressive (misses some  
coreferences) it is arguably simpler.

Cheers,
Bijan.