anchor.diff:  open FsUnit
   open FsKarel.Core
   
  -let testFail() = true |> should be False
  -let testSuccess() = true |> should be True
  +let testFail a = true |> should be False
  +let testSuccess a = true |> should be True
  +let assertResult r = either testSuccess testFail
   
   [<Test>]
   let ``Addition of walls to a world should work``() =
  
positive.code: [<Test>]
  let ``Addition of beepers to a world should work``() =
    let world = World.Default
    world.beepers |> should equal Map.empty
  
    let pos = (3u, 3u)
    let result = World.setBeepers 5u pos world
    match result with
    | Failure _ -> testFail()
    | Success worldWithBeepers ->
      Map.containsKey pos worldWithBeepers.beepers |> should be True
      let beepers = Map.find pos worldWithBeepers.beepers
      beepers |> should equal 5u
  
      let result2 = World.setBeepers 8u pos worldWithBeepers
  
      match result2 with
      | Failure _ -> testFail()
      | Success worldWithBeepers2 ->
        Map.containsKey pos worldWithBeepers2.beepers |> should be True
        let beepers = Map.find pos worldWithBeepers2.beepers
  
        beepers |> should equal 8u
    
  [<Test>]
  let ``Adding a beeper outside of the world should testFail``() =
    let world = World.Default
    
    let pos = (200u, 200u)
    let result = World.setBeepers 5u pos world
    match result with 
    | Failure _ -> testSuccess()
    | Success _ -> testFail()
    
  [<Test>]
  let ``hasBeeper should work``() =
    let world = World.Default
    let pos = (3u, 3u)
    match World.hasBeepers pos world with 
      | Failure _ -> testFail()
      | Success r -> r |> should be False
  
negative.code:     world
  
    let addWallNorth = (World.addWall WallPositions.North pos)
    let assertWallNorth = map (assertAddingWalls WallPositions.North)
    let addWallSouth = bind (World.addWall WallPositions.South pos)
    let assertWallsNorthAndSouth = map (assertAddingWalls (WallPositions.North ||| WallPositions.South))
  
    world
      |> addWallNorth
      |> assertWallNorth
      |> addWallSouth
      |> assertWallsNorthAndSouth
      |> assertResult
      |> ignore
    
  [<Test>]
  let ``Adding a wall outside of the world should testFail``() =
    let world = World.Default
    let pos = (200u, 200u)
    
    World.addWall WallPositions.North pos world |> either testFail testSuccess
  
  [<Test>]
  let ``hasWall should work``() =
  