anchor.diff:          | Some result ->
               result
           | None ->
  +            // TODO : Return a better error message
  +            //keyNotFound ""
               raise <| System.Collections.Generic.KeyNotFoundException ()
   
   
  
positive.code: 
          match resizeArray.FindIndex (System.Predicate predicate) with
          | -1 ->
              raise <| System.Collections.Generic.KeyNotFoundException ()
          | index ->
              resizeArray.[index]
  
negative.code: 
          match resizeArray.FindIndex (System.Predicate predicate) with
          | -1 ->
              // TODO : Add a better error message.
              // keyNotFound ""
              raise <| System.Collections.Generic.KeyNotFoundException ()
          | index ->
              index
  