(define (enclosing-rect rects)
  (when (null? rects)
    (abort (usage-error "RECTS must be a list of at least one rect"
                        'enclosing-rect)))
  (let loop ((x (rect-x (car rects)))
             (y (rect-y (car rects)))
             (w (rect-w (car rects)))
             (h (rect-h (car rects)))
             (rects (cdr rects)))
    (if (pair? rects)
        (let ((rect (car rects)))
          (loop (min (rect-x rect) x)
                (min (rect-y rect) y)
                (max (rect-w rect) w)
                (max (rect-h rect) h)
                (cdr rects)))
        (rect x y w h))))

(define (enclosing-rect rects)
  (when (null? rects)
    (abort (usage-error "RECTS must be a list of at least one rect"
                        'enclosing-rect)))
  (fold (lambda (current outer)
          (let ((x (min (rect-x current) (rect-x outer)))
                (y (min (rect-y current) (rect-y outer)))
                (w (max (rect-w current) (rect-w outer)))
                (h (max (rect-h current) (rect-h outer))))
            (rect x y w h)))
        (car rects) (cdr rects)))

(define (extend-bounding-box r1 r2)
  (let ((x (min (rect-x r1) (rect-x r2)))
        (y (min (rect-y r1) (rect-y r2)))
        (w (max (rect-w r1) (rect-w r2)))
        (h (max (rect-w r1) (rect-w r2))))
    (rect x y w h)))

(define (enclosing-rect rects)
  (when (null? rects)
    (abort (usage-error "RECTS must be a list of at least one rect"
                        'enclosing-rect)))
  (fold extend-bounding-box (car rects) (cdr rects)))