It does not work correctly even for the most simple sentences such as “the professor lectures”. For the above sentence, the parser can produce the correct result when it runs for the first time. But if one runs try-again, it will be stuck in an infinite loop because the evaluation of (parse-prepositional-phrase) always fails. The failure will cause the evaluation of (list 'verb-phrase (parse-verb-phrase) (parse-prepositional-phrase)), which in turn will cause the the evaluation of (parse-prepositional-phrase) again. Moreover, if the sentence contains a misspelled word, the parser will be stuck too.
Changing the order of expressions in the amb would make things even worse. It would fail for the above simple sentence even in the first round.
