Testing HsColour. Here you go: fibonacci with iso-recursive types

> data Mu f = Mu {unMu :: f (Mu f)}
> type List a = Mu ((,) a)
> tl = snd . unMu
> hd = fst . unMu
> cns = (Mu .) . (,)
> ones = Mu (1,ones)
> fibs :: List Integer
> fibs = cns 1 . cns 1 $ go fibs (tl fibs)
>        where go (Mu (x,xs)) (Mu (y,ys)) = cns (x+y) $ go xs ys

(NB. planet readers, you can't see the colours)