Natural Transformations
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Let \\(F\\) and \\(G\\) be two functors from \\(\\C\\) to \\(\\D\\).

<img src="categories/img/mp/natural-transformation.png" alt="Natural transformation commutative diagram" class="right"/>
_A natural transformation:_ familly η ; \\(η\_X\\in\\hom{\\D}\\) for \\(X\\in\\ob{\\C}\\) s.t.

ex: between Haskell functors; `F a -> G a`  
Rearragement functions only.