(define part_sum (lambda (S) (part_sum2 S '() '())))

(define part_sum2 (lambda (S A B)
  (if (pair? S)
      (max (part_sum2 (cdr S) (cons (car S) A) B)
         (part_sum2 (cdr S) A (cons (car S) B)))
      (if (= (apply + A) (apply + B))
          1
          0))))