sub printcats {
   my ($class, $tree) = @_;

   my $printsub = sub {
                         my $node = shift;
                         print $node->{data}, "\n";
                      };

   $tree->traverse($tree,
                   $Tree::Nary::PRE_ORDER,
                   $Tree::Nary::TRAVERSE_ALL,
                   -1,
                   $printsub
                  );
}