Proposal: containers: add indexing operations for Set

[Milan Straka]

Hi,

> I'd like to add the following indexing operations for Data.Set.Set, to
> complement the existing functions for Data.Map.Map:
>
> findIndex :: Ord a => a -> Set a -> Int
> lookupIndex :: Ord a => a -> Set a -> Maybe Int
> elemAt :: Int -> Set a -> a
> deleteAt :: Int -> Set a -> Set a

+1

Milan