Proposal: containers: add indexing operations for Set

[Henning Thielemann]

On Wed, 4 Jul 2012, Thomas Schilling wrote:

> I'm surprised that these functions already exists in Data.Map.  What
> are the use cases?

These index functions should certainly be part of an OrderedMap AP. But 
since Map functions have "Ord" constraints and not "MapDomain", the 
abstraction is already broken by the way how Data.Map is designed today.

Anyway, I found the index functions useful when working with wxHaskell and 
Notebook pages. WX manages these pages by numbers, but I want to add and 
remove pages dynamically while keeping the pages sorted with respect to 
their titles.