crop

;;; (crop img left top width height [description]) -> image?
;;;   img : image?
;;;   left : real?
;;;   top : real?
;;;   width : nonnegative-real?
;;;   height : nonnegative-real?
;;;   description : string?
;;; Crop `img` with the left edge at `left`, the top edge at `top`,
;;; and the specified width and height.

> (define disc (solid-circle 100 "red"))
> disc

> (crop disc 0 0 50 50)

> (crop disc 50 50 50 50)

> (crop disc -10 0 50 50)

> (crop disc 0 20 50 50)