[O] Narrow to sparse tree

[Christopher Witte]

I keep a bibliography in an org file using the excellent org-bibtex
functionality.  It has over 100 entries with a first level heading for
each item.  I tag each of the items with keywords/topics.  Using
sparse trees I can quickly find and navigate to items matching a
particular tag, but if only 4 items out of the 100 have that tag the
"signal to noise" is pretty bad.  What I would like to do is narrow
the buffer to just those entries that match the tag.  I've looked in
the manual but I couldn't find a way to do this.  Can this be done
with org?

Thanks for the help,
Chris.

ps. using M-g n or M-g M-n (next-error) with sparse trees I expected
it to wrap when you reach the end of the file but sadly this is not
the case.  Is there a reason for this?