circle?

;;; (circle? img) -> boolean?
;;;   img : image?
;;; Determine if the image is a circle.

(define circle?
  (lambda (img)
    (or (solid-circle? img)
        (outlined-circle? img)
        (and (ellipse? img)
             (<= (abs (- (ellipse-width img) (ellipse-height img))) 0.01))
        (and (transformed? img)
             (preserved? img)
             (circle? (subimage img))))))