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 action
  
      //
      [<CompiledName("IterateBack")>]
      let inline iterBack (action : int -> 'T -> unit) (map : IntMap<'T>) =
          // Preconditions
          checkNonNull "map" map
  
negative.code:         |> retype
  
      //
      [<CompiledName("Iter")>]
      let inline iter (action : int<'Tag> -> 'T -> unit) (map : TagMap<'Tag, 'T>) : unit =
          // Retype as IntMap.
          let map : IntMap<'T> = retype map
  