[Haskell-beginners] mapKeysMaybe

[Dennis Raddle]

Can someone give me a clever way to implement mapKeysMaybe, defined as
follows?

Ord k => (k -> Maybe k) -> Map k a -> Map k a

where the transformed keys that evaluate to Just are the only ones kept?

I am thinking this will probably help me understand a new type, like
traversable or something.
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