; Return a list of URLS as located in the a elements of ast.  
(define (extract-links ast)
 (if (ast? ast)
     (let ((name (ast-element-name ast))
           (subtrees (ast-subtrees ast))
          )
       (if (equal? name "a")
           (let ((href-attr-value 
                  (find-href-attribute (ast-attributes ast))))
             (if href-attr-value (list href-attr-value) '()))
           (extract-links-ast-list subtrees)))
     '()))

; Return a list of URLS as located in the a elements of 
; the list of ast's as passed in ast-list.  
(define (extract-links-ast-list ast-list)
 (if (null? ast-list)
     '()
     (append 
      (extract-links (car ast-list))
      (extract-links-ast-list (cdr ast-list)))))