Rang-(2/N)-Polymorphie in GHC

http://haskell.org/ghc/docs/latest/html/users_guide/other-type-extensions.html

{-# language Rank2Types #-}
f2 :: (forall a. a->a) -> Int -> Int
f4 :: Int -> (forall a. a -> a)

{-# language RankNTypes #-}
f3 :: ((forall a. a->a) -> Int) 
    -> Bool -> Bool

Rang: 1 + max. Tiefe der ($\Box$$\to$*)-Kontexte für Typ-Forall