while (my (@positions) = $i->()) {
   my @tmp;
   my $cur = 0;
   for my $pos (@positions) {
      push @tmp, @arr[$cur..$pos-1];
      $cur = $pos;
      push @tmp, 'N';
   }
   push @tmp, @arr[$cur..$#arr];
   print(join(',', @tmp), "\n");
}