{
   #      a
   #     / \
   #    b   e
   #   / \   \
   #  c   d   f

   my $tree = [
      'a',
      [
         'b',
         [
            'c',
            undef,
            undef,
         ],
         [
            'd',
            undef,
            undef,
         ], 
      ],
      [
         'e',
         undef,
         [
            'f',
            undef,
            undef,
         ], 
      ], 
   ];

   my $iter = make_iter(\&visit_preorder, $tree);
   while (my ($name) = $iter->()) {
      print($name);
   }
   print("\n");
}