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                sort { $a->[1] <=> $b->[1] }
                map  { $arr[$_] >= 0 ? [ $_, max_prime_factor($arr[$_]
+) ] : () }
                0 .. $#arr

                sort { max_prime_factor($arr[$a]) <=> max_prime_factor
+($arr[$b]) }
                grep { $arr[ $_ ] >= 0 }
                0 .. $#arr