Add everywhereM' to Data.Generics.Schemes

[Maciej Podgurski]

Hello,

as an analogue to everywhere', I suggest adding everywhereM' to 
Data.Generics.Schemes. The implementation would be

    everywhereM' :: Monad m => GenericM m -> GenericM m
    everywhereM' f x = do { x' <- f x;
                            gmapM (everywhereM' f) x' }


Best wishes,

Maciej