compound-image?

;;; (compound-image? val) -> boolean?
;;;   val : any?
;;; Determines if `val` is a compound image, one build by combining
;;; other images (e.g., with `overlay`, `beside`, or `above`).

> (compound-image? (overlay (solid-circle 10 "blue")
                            (solid-square 10 "gray")))
#t
> (compound-image? (solid-circle 10 "blue"))
#f
> (compound-image? (rotate (solid-square 10 "gray") 30))
#f
> (compound-image? 23)
#f
> (compound-image? (beside (solid-circle 10 "blue")
                           (outlined-circle 9 "blue" 1)))
#t