Prog challenge:

Write Ascii85 to binary converter in 10 lines of code.
Hard Mode: Using Python.
Hardcore Mode: in Esoteric language of your choice.

>>27
(***) :: a b c -> a b' c' -> a (b, b') (c, c')
I think this one is pretty clear.
Consider reading appropriate papers¹ for fuller explanation.
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¹ http://www.haskell.org/haskellwiki/Research_papers/Monads_and_arrows#Arrows