[pseudonom-]

As a sort of side note, why is it

    filter p (x :: xs) with (filter p xs)
        | (_ ** xs') = if p x then (_ ** x :: xs') else (_ ** xs')

rather than

    filter p (x :: xs) = if p x then (_ ** x :: snd (filter p xs)) else (_ ** snd (filter p xs))

?