#lang slideshow
(define (square n)
  ; A semi-colon starts a line comment.
  ; The expression below is the function body.
  (filled-rectangle n n))

(define (checker p1 p2)
  (let ([p12 (hc-append p1 p2)]
        [p21 (hc-append p2 p1)])
    (vc-append p12 p21)))

> (checker (colorize (square 10) "red")
           (colorize (square 10) "black"))

(define (four p)
  (define two-p (hc-append p p))
  (vc-append two-p two-p))

> (four (circle 10))

(define (checkerboard p)
  (let* ([rp (colorize p "red")]
         [bp (colorize p "black")]
         [c (checker rp bp)]
         [c4 (four c)])
    (four c4)))

> (checkerboard (square 10))