How do I output only a subset of a graph?

[Matthew Cornell]

Hi Folks. I have a graph computation that starts with a subset of vertices
of a certain type and propagates information through the graph to a set of
target vertices, which are also subset of the graph. I want to output only
information from those particular vertices, but I don't see a way to do
this in the various VertexOutputFormat subclasses, which all seem oriented
to outputting something for every vertex in the graph. How do I do this?
E.g., are there hooks for the output phase where I can filter output? Or am
I supposed to write a VertexOutputFormat implementation that generates no
output for the vertices that have no data? Thanks in advance.

-- 
Matthew Cornell | m...@matthewcornell.org | 413-626-3621 | 34 Dickinson
Street, Amherst MA 01002 | matthewcornell.org