data Apa a = XX a
           | YY (b, b -> Int)

f (XX a)     = a
f (YY (c,f)) = f c

apa  = YY ("apa", length)
apa' = XX 7

data Bepa > Apa (a,a->Int) = ZZ

g ZZ         = 0
g (YY (c,f)) = f c
g (XX (c,f)) = f c
-- g (XX a)  = a

bepa = XX (False, const 1)