(define (bubble sent)
  (cond ((empty? (butfirst sent)) sent)
	((<= (first sent) (first (bf sent)))
	 (se (first sent) (bubble (bf sent))))
	(else (se (first (bf sent))
		  (bubble (se (first sent) (bf (bf sent))))))))

(define (sort sent)
  (if (empty? sent)
      '()
      (let ((bub (bubble sent)))
	(se (sort (butlast bub))
	    (last bub)))))