lolo-iterator should be more robust

[bel28kent]

In lines 83–94, the helper would be more robust if it checked for both lolot and lolon to be empty. If only one is empty, it should throw an exception.

          ; (listof (listof Token)) (listof (listof Natural)) -> (listof (listof (listof Token)))
          (define (lolo-iterator lolot lolon)
            ; spine-list: (listof (listof Token)). Accumulates the output of lon-iterator calls.
            ;
            (local [(define (lolo-iterator lolot lolon spine-list)
                      (cond [(andmap empty? lolot) (reverse spine-list)]
                            [else
                              (local [(define output (lon-iterator lolot (first lolon)))]
                                (lolo-iterator (acc-remaining output)
                                               (rest lolon)
                                               (cons (acc-spine-list output) spine-list)))]))]
              (lolo-iterator lolot lolon empty)))