anchor.diff:          | Some x -> x
           | None ->
               // TODO : Add a better error message which includes the key.
  +            //keyNotFound ""
               raise <| System.Collections.Generic.KeyNotFoundException ()
   
       /// Returns a new IntMap with the binding added to this IntMap.
  
positive.code:         map.Iterate (retype action)
  
      //
      [<CompiledName("IterateBack")>]
      let inline iterBack (action : int<'Tag> -> 'T -> unit) (map : TagMap<'Tag, 'T>) : unit =
          // Retype as IntMap.
          let map : IntMap<'T> = retype map
  
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."
  