(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)))