Pierre

in [prelude](http://www.preludels.com/#scanr) invalid signature of `scanr` specified Now: `(a → b → b) → b → [a] → [b]` Should be: `(b → a → b) → b → [a]...

I suggest to add the functions: ``` mapAccumL :: (a -> b -> (a, c)) -> a -> [b] -> (a, [c]) mapAccumR :: (a -> b -> (a, c))...