Testing: modify parse to just take a verb phrase

This was done to test the alternative parse-verb-phrase.

mceval/amb-natural-language.rkt

@@ -23,7 +23,7 @@
 
 (define (parse input)
   (set! *unparsed* input)
-  (let ((sent (parse-sentence)))
+  (let ((sent (parse-verb-phrase 0)))
     (require (null? *unparsed*))
     sent))