Lift a traversal over a kind-indexed type to a traversal over a type that existentially wraps it

ExistentialTraversal.hs

someCard :: forall b f. Applicative f 
         =>  (forall f q r t. Applicative f => (b -> f b) 
                                            -> Card q r t 
                                            -> f (Card q r t)) 
         -> ((b -> f b) -> SomeCard -> f SomeCard)  
someCard t = \_ -> h (g t)
  where 
    g :: forall b f. Applicative f 
      => (forall q r t . (b -> f b) -> Card q r t -> f (Card q r t)) 
      -> (forall q r t. Card q r t -> f (Card q r t))
    g f = f (pure . identity)

    h :: forall f. Applicative f 
      => (forall q r t.  (Card q r t -> f (Card q r t))) 
      -> (SomeCard -> f SomeCard)
    h f = \(MkSomeCard c) -> MkSomeCard <$> f c