[boost] Has anyone thought about adding a general case subset class?

I've got a lot working with my madlib version of graph, but I've been thinking over the elegant ways to add the subgraph feature in BGL.  It occurred to me that this can be phrased as a more general problem as container subsets.  And while we're at that, then we can add objects or operators for set operations like inner join, left join, and outer join. Can I get some thoughts?