anchor.diff:          | _ -> this
   
   type FieldSpace =
  -    { PickupItemOpt : PickupType Option }
  +    { CharacterOpt : CharacterIndex Option
  +      PickupItemOpt : PickupType Option }
  +
  +    member this.ContainsCharacter =
  +        match this.CharacterOpt with Some _ -> true | None -> false
   
       member this.ContainsPickup =
           match this.PickupItemOpt with Some _ -> true | None -> false
       
  +    static member updateCharacterOpt updater fieldSpace =
  +        { fieldSpace with CharacterOpt = updater fieldSpace.CharacterOpt }
  +    
       static member updatePickupItemOpt updater fieldSpace =
           { fieldSpace with PickupItemOpt = updater fieldSpace.PickupItemOpt }
       
  +    static member addCharacter index fieldSpace =
  +        FieldSpace.updateCharacterOpt (constant (Some index)) fieldSpace
  +
  +    static member removeCharacter fieldSpace =
  +        FieldSpace.updateCharacterOpt (constant None) fieldSpace
  +    
       static member addHealth fieldSpace =
           FieldSpace.updatePickupItemOpt (constant (Some Health)) fieldSpace
       
  
positive.code:     static member pickupAtCoordinates coordinates chessboard =
          chessboard.PassableCoordinates.[coordinates].ContainsPickup
      
      static member updateCoordinatesValue updater coordinates chessboard =
          let passableCoordinates = Map.add coordinates (updater chessboard.PassableCoordinates.[coordinates]) chessboard.PassableCoordinates
          Chessboard.updatePassableCoordinates (constant passableCoordinates) chessboard
      
      static member clearPickups _ _ chessboard =
          let passableCoordinates = Map.map (fun _ v -> FieldSpace.removePickup v) chessboard.PassableCoordinates
  
negative.code:             Chessboard.updateCharacterCoordinates (constant characterCoordinates) chessboard
          else failwith "character placement failed; coordinates unavailable"
  
      static member addCharacter index coordinates (chessboard : Chessboard) =
          Chessboard.placeCharacter index coordinates chessboard
  
      static member relocateCharacter index coordinates (chessboard : Chessboard) =
          Chessboard.placeCharacter index coordinates chessboard
      
      static member removeCharacter index _ chessboard =
          let characterCoordinates = Map.remove index chessboard.CharacterCoordinates
          Chessboard.updateCharacterCoordinates (constant characterCoordinates) chessboard
  