More Data.Set and Data.Map operations

[Henning Thielemann]

On Sun, 7 Aug 2016, David Feuer wrote:

> data MergeTactic k x y where
>  Drop :: MergeTactic k x y
>  Preserve :: MergeTactic k x x
>  MapFilter :: (k -> x -> Maybe y) -> MergeTactic k x y
>
> generalMerge :: Ord k
>             => (k -> a -> b -> Maybe c) -> MergeTactic k a c ->
> MergeTactic k b c
>             -> Map k a -> Map k b -> Map k c
>
> How do people feel about exporting this for GHC?

If I import a package (containers) with functions that are only available 
on GHC then I need to insert conditionals for whether I am running on GHC 
or not.