They're very hard, as a normal human, I can't defeat them without some tools, such as pencil, paper, etc..
And the most important, you passed the 16 bonus levels at last, there's no achievement...
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I can't finish the ++, so there's only +'s 8 levels
The ++ seems only can be beaten by brute-force attack with method of exhaustion, forgive me if someone find a better way.