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I think it's still pretty unclear what this function might do, since the inverse of a map isn't well-defined. I think this is a type signature that makes sense and has a fairly obvious most-likely meaning:

    boo :: Map a b -> b -> Set a
I'm not sure how you would conjure up just an 'a' in general.

You also need an Ord or at least an Eq constraint on the b's.

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