twice f x = f (f x)

-- Does not pass the type checker due to occur check
--fun g = g g

len []     = 0::Int
len (x:xs) = 1 + len xs

len2 xs ys = len xs + len ys

mainlen2 = len2 [1,2,3] "Hello"


-- Arbitrary recursive functions:
f n = g n + h n
g n = j n + f n
h n = i n + g n
i n = n
j n = k n
k n = j n + l n
l n = 5 :: Int