sub repeated_substring {
   my ($ssl,$pos) = (1,-1);
   my $len = length($_[0]);
   my $i = 0;
   while ($i < $len-2*$ssl) {
      if (index($_[0],substr($_[0],$i,$ssl),$i+$ssl) == -1) { $i++; next }
      $pos = $i; $ssl++;
   }
   return $pos == -1 ? "" : substr($_[0], $pos, $ssl-1);
}