sub ackermann
{
   my ($n, $m, $seen) = @_;
   
   $seen = {} unless defined $seen;

   while(1)
   {
       if($n == 0)
       {
       	   $seen->{"$n,$m"} = $m+1;       	   
           return ($m+1);
       }
       elsif($m == 0)
       {
           $n--;
           $m = 1;
       }
       else
       {
           my $z = $seen->{"$n,".($m-1)};       	   
           $m = $z || ackermann($n, $m-1, $seen);
           $n--;           
       }
   }
}