FiniteMap: modifyFM

[Johannes Waldmann]

> Similar for Sets, lists, and FMs, I'd expect something like 
> 
>         instance Collection a where
>                 empty = ...
>                 (!!) 
>                 cardinality -- or whatever

There is also the option of separating things even further,  like

    class Size a where size :: a -> Int
    class Empty a where empty :: a

ultimately each function is the (single) method of a class. 

for accessing an indexed collection , write

    class Bang c where (!!) : c a b -> a -> b

(this is AssocX.lookup in Edison, 
http://www.haskell.org/ghc/docs/edison/users008.html#toc35)

even better, since this is actually a (partial) mapping, 
change (!!) to ($), and we have unified notation for
functions, arrays, lists, finite maps... 

not entirely joking,
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