Question 12 is unfair

[dmison]

Up until Q12 it has been little steps, introducing new things, then combining them. Q12 combines a number of previous concepts but also assumes users will realise to use closures to reference values in previous operations.

This is unfair to the learner. You should never have questions that require specific knowledge that you either haven't previously used in exercises or or at least referenced.

I would suggest at least one preceding simpler exercise with "nested" maps where the inner one references the outer. Then you can through this complex one at them.

[dariusvertex]

I also got stuck on this question and had no ideas besides viewing the answer. After viewing answer I found out new idea that I can use upper value.
So there should be at least example of this technique before doing this excersize.