finding the leaf packages

["Steven M. Bellovin" <smb%cs.columbia.edu@localhost>]

Is there some simple way to get a list of all leaf packages, in the form
of, say, pkg_tools/pkgdepgrah.  (I use that as a non-random example, since
it obviously knows how to do that, but I haven't figured out the options
to do it in any way other than as input to graphviz.)

                --Steven M. Bellovin, http://www.cs.columbia.edu/~smb