anchor.diff:                      mapRec (result :: acc, tl1, tl2)
   
               | _, _ ->
  -                failwith "The lists have different lengths."
  +                invalidArg "list2" "The lists have different lengths."
                           
           // Call the recursive implementation function. 
           mapRec (List.empty, list1, list2)
  
positive.code:                 | Choice1Of2 false ->
                      existsRec (tl1, tl2)
              | _, _ ->
                  failwith "The lists have different lengths."
  
          // Call the recursive implementation function.
          existsRec (list1, list2)
  
negative.code:                 | Choice1Of2 true ->
                      existsRec (tl1, tl2)
              | _, _ ->
                  invalidArg "list2" "The lists have different lengths."
  
          // Call the recursive implementation function.
          existsRec (list1, list2)
  