anchor.diff:          /// <inherit />
           member __.Keys
               with get () =
  +                // TODO : Return the keys as an ICollection<int>
                   notImpl "IDictionary`2.Keys"
   
           /// <inherit />
           member __.Values
               with get () =
  +                // TODO : Return the values as an ICollection<'T>
                   notImpl "IDictionary`2.Values"
   
           /// <inherit />
  
positive.code:     let inline singleton (key : int) (value : 'T) : IntMap<'T> =
          IntMap.Singleton (key, value)
  
      /// Look up an element in the IntMap returning a Some value if the
      /// element is in the domain of the IntMap and None if not.
      [<CompiledName("TryFind")>]
  
negative.code:                 | Some value ->
                      value
                  | None ->
                      // TODO : Provide a better error message here.
                      //keyNotFound ""
                      raise <| System.Collections.Generic.KeyNotFoundException ()
              and set key value =
                  notSupported "IntMaps cannot be mutated."
  