What is the meaning of Adjustable?

[parenthetical]

It looks like something I could make use of but I'm having a hard time wrapping my head around it.

Would it be something like this for the pure implementation?

instance ( Enum t, HasTrie t, Ord t, t' ~ Pure t
         ) => Adjustable (Pure t) ((->) t) where
  runWithReplace :: (t -> a) -> Event t' (t -> b) -> t -> (a, Event t' b)
  runWithReplace ma (Event e) from = (ma from, Event (fmap ($ from) . e))