(define (grow-huffman-tree pairs)
  (define (make-tree leaf-set)
    (cond 
      ((null? leaf-set) '())                          ; no element
      ((null? (cdr leaf-set)) (car leaf-set))         ; one element
      (else                                           ; two elements
       (let ((first (car leaf-set))
             (second (cadr leaf-set))
             (rest (cddr leaf-set)))
         (if (null? rest)
             (make-code-tree first second)
             (make-code-tree (make-code-tree first second)
                             (make-tree rest)))))))
  (make-tree (make-leaf-set pairs)))