at FilePath is traversed/descended
        into.  It will be called with each path for which C{isdir} returns
        C{True}.  If C{descend} is not specified, all directories will be
        traversed (including symbolic links which refer to directories).

        @param descend: A one-argument callable that will return True for
            FilePaths that should be traversed, False otherwise.

        @return: a generator yielding FilePath-like objects.
        Nz