anchor.diff:                      iterRec (tl1, tl2, index + 1)
   
               | _, _ ->
  -                failwith "The lists have different lengths."
  +                invalidArg "list2" "The lists have different lengths."
                           
           // Call the recursive implementation function. 
           iterRec (list1, list2, 0)
  
positive.code:                     foldRec (tl1, tl2, state)
  
              | _, _ ->
                  failwith "The lists have different lengths."
                          
          // Call the recursive implementation function. 
          foldRec (list1, list2, state)
  
negative.code:                     mapRec (result :: acc, index + 1, tl1, tl2)
  
              | _, _ ->
                  invalidArg "list2" "The lists have different lengths."
                          
          // Call the recursive implementation function. 
          mapRec (List.empty, 0, list1, list2)
  