struct Apa a =
   xx :: a -> a
   yy :: b -> b
   

struct Bepa < Apa a =
   zz :: Int

f s = (s.yy 'X', s.yy True)

apa x = struct
         xx = \a -> x
         yy = \b -> b

bepa x = struct
         xx = \a -> a
         yy = \b -> b     -- \b -> x
         zz = 1

g s = (s.xx 'X', s.xx True, s.zz)