(define (encode message tree)
  (define (encode-helper current-message left right)
    (define (check-word leaf word)
      (if (equal? (symbol-leaf leaf) word)
          #t
          #f))
    (cond ((leaf? right) (check-word right (car current-message)))
          ((leaf? left) (check-word left (car current-message)))
          (else (encode-helper current-message (left-branch left) (right-branch right)))))
  (encode-helper message (left-branch tree) (right-branch tree)))