import GHC.Generics

data TNothing = TNothing
data TJust a = TJust a

class Something tag f where
    something :: f p -> tag

instance (Something TNothing f, Something tag g) => Something tag (f :*: g) where
    something (_ :*: b) = something b

instance (Something tag f, Something TNothing g) => Something tag (f :*: g) where
    something (a :*: _) = something a