(define (filter p? xs)
  (cond ((null? xs) '())
        ((p? (car xs))
           (cons (car xs) 
                 (filter p? (cdr xs))))
        (else (filter p? (cdr xs)))))

(define (sort f xs)
  (if (null? xs) '()
      (let* ((pivot (car xs))
             (left (filter (lambda (x) (f x pivot)) (cdr xs)))
             (right (filter (lambda (x) (not (f x pivot))) (cdr xs))))
        (append (sort f left) (list pivot) (sort f right)))))