That really helps. Kind of like a lazy evaluating ST.

The problem remains O9n*n!), but for n-1 * n! passes the comp() comes down to

    return $cache{$_[0]}{$_[1]} 
        if exists $cache{$_[0]}{$_[1]};
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Which is a huge time saver (provided I don't run out of memory:).

Thanks.