while (my (@positions) = $i->()) {
   my $sep = '';
   my $cur = 0;
   for my $pos (@positions) {
      print($sep);
      $sep = ', ';
      print($arr[$cur++], $sep) while $cur < $pos;
      print('N');
   }
   print($sep, $arr[$cur++]) while $cur < @arr;
   print("\n");
}