-unknown variables, which makes restoring the file much more
-difficult@FIXME-xref{how to extract sparse file using third-party @command{tar}s}.
+unknown variables, which makes restoring the file more
+difficult. @xref{extracting sparse v.0.x, Extraction of sparse
+members in v.0.0 format}, for the detailed description of how to
+restore such members using non-GNU @command{tar}s.