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#lang racket

(require racket/future)

(define (sum values)
  (foldr + 0 values))

(define (best vals-1 vals-2 target)
  (if (< (abs (- target (sum vals-1)))
         (abs (- target (sum vals-2))))
      vals-1
      vals-2))

(define (closest values target)
  (cond [(= 1 (length values))
         values]
        [(= 0 target)
         '()]
        [else
         (let ([wout-first
                (future () (closest (rest values) target)))]
               [with-first 
                (future () (closest (rest values) (- target (first values)))))])
           (best (touch wout-first) (cons (first values) (touch with-first)) target))]))