sub increment_odometer {
   my @odometer = @_;
   my $wheel = $#odometer; # start at rightmost wheel
   until ($odometer[$wheel] < 9 || $wheel < 0) {
      $odometer[$wheel] = 0;
      $wheel--; # next wheel to the left
   }
   if ($wheel < 0) {
   return; # fell off the left end; no more sequences
   } 
   else {
         $odometer[$wheel]++; # this wheel now turns one notch
         return @odometer;
   }
}