[Haskell-beginners] Foldable of Foldable

[Julian Birch]

Apologies if this isn't clear, I suspect if I understood the terminology
better I'd already have solved the problem. I've got a foldable of a
foldable of a.  (Specifically `[Set a]`) and I want to be able to express
the concept that I can treat `(Foldable f1, Foldable f2) => f1 (f2 a)` as
`f3 a` i.e. a foldable of a foldable of a can be newtyped to a foldable of
a.  At least, I think that's right.

Sadly, my attempts at actually expressing this concept have met with
incomprehension from GHC, could someone help me out, please?

Thanks,

Julian.
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