Library proposal: add a Location interface for element-wise operations on Data.Map (#4887)

[Henning Thielemann]

On Sat, 8 Jan 2011, Ross Paterson wrote:

> That would work well for search, but then index, minLocation and maxLocation
> would return Locations that value was always mapped to Just something.
> Extra invariants like that feel wrong to me.

'index' can be out of range and then return Nothing. minLocation and 
maxLocation return Nothing if the Map is empty.