And of course it is fun to think about total functions that are "effectively" bottom...

    id :: a -> a
    id a = case (a,ack 4 3) of
        (x,0) -> x
        (x,_) -> x
        where
            ack 0 n = n+1
            ack m 0 = ack (m-1) 1 
            ack m n = ack (m-1) (ack m (n-1))