cons :: (a → a) → listf → listf
dapply :: a → (a → a) → (a → a) → a
lapply :: a → listf → a
nil :: listf
dapply(x, F, G) → F(G(x))
lapply(x, nil) → x
lapply(x, cons(F, lf)) → F(lapply(x, lf))