sub using_index {
   our $seq; *seq = \$_[0];
   my @groups;

   my $pos = -1;
   my $start = -1;

   for (;;) {
      my $new_pos = index($seq, 'M', $pos+1);

      if ($new_pos < 0) {
         if (defined($start)) {
            push(@groups, [ $start, $pos ]);
         }
         last;
      }

      if ($start < 0) {
         $start = $new_pos;
      }
      elsif ($new_pos - $pos > 1) {
         push(@groups, [ $start, $pos ]);
         $start = $new_pos;
      }

      $pos = $new_pos;
   }

   return @groups;
}