[racket] documentation example for slideshow/pict

Here's a better example for slideshow/pict, a new version of
`print-sudoku'.  Racket sounds like a very interesting ambition
attempt to ``to fuse cutting-edge programming-language research with
everyday programming.''  I don't know how far along the project is,
but eventually you'll need good documentation, which means examples.

#lang slideshow

;; Digit = 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
;; NL = number or list = Digit | (list of Digits)
;; Sudoku = (list of list of NL)

(define (digit->char i)
  (integer->char (+ 48 i)))

(define (Build-list n f) (build-list n (lambda (i) (f (add1 i)))))    

;; print-sudoku: Sudoku -> Pict
;; to build a picture of a sudoku as a 9x9 table, with all 9 boxes boxed, with a text picture for each big number, and a 3x3 table of text pictures for each empty cell list.
(define (print-sudoku S) 
  (define (NL->pict nl)
    (if (list? nl)
        (table 3 (Build-list 9 (lambda (i) 
                                 (if (member i nl)
                                     (text (make-string 1 (digit->char i)) 
                                           "Courier" 16 0)
                                     (text " " "Courier" 16 0))))
               (cons cc-superimpose cc-superimpose) 
               (cons cc-superimpose cc-superimpose)
               (cons 0 0) (cons 0 0))
        (inset (t (list->string (list  (digit->char nl)))) 20 0)))
  
  (let* ([unboxed-table (table 9 (map NL->pict (apply append S))
                               (cons cc-superimpose cc-superimpose) 
                               (cons cc-superimpose cc-superimpose)
                               (cons 0 0) (cons 10 10))]
         [pw (pict-width unboxed-table)]
         [P (pict-height unboxed-table)]
         [thick-box-lines 
          (linewidth 7 (lt-superimpose 
                        (rectangle pw ph)
                        (rectangle (/ pw 3) ph) (rectangle (* 2 (/ pw 3)) ph)
                        (rectangle pw (/ ph 3)) (rectangle pw (* 2 (/ ph 3)))))])
    (lt-superimpose 
     unboxed-table thick-box-lines
     (rectangle (* pw (/ 1 9)) ph) (rectangle pw (* ph (/ 1 9))) 
     (rectangle (* pw (/ 2 9)) ph) (rectangle pw (* ph (/ 2 9))) 
     (rectangle (* pw (/ 3 9)) ph) (rectangle pw (* ph (/ 3 9))) 
     (rectangle (* pw (/ 4 9)) ph) (rectangle pw (* ph (/ 4 9))) 
     (rectangle (* pw (/ 5 9)) ph) (rectangle pw (* ph (/ 5 9))) 
     (rectangle (* pw (/ 6 9)) ph) (rectangle pw (* ph (/ 6 9))) 
     (rectangle (* pw (/ 7 9)) ph) (rectangle pw (* ph (/ 7 9))) 
     (rectangle (* pw (/ 8 9)) ph) (rectangle pw (* ph (/ 8 9))))))

(define Unsolvable#7
  '((8 (3 7) (3 5 7) 2 (3 5) 1 9 4 6)
    (2 4 6 (7 8) 9 (5 7 8) (5 7) 1 3)
    ((3 5) 1 9 (3 7) 4 6 (5 7) 8 2)
    (1 2 4 (3 6 8) (3 8) (5 7) (3 6) (5 7) 9)
    ((3 5 7) (3 6 7) 8 9 2 4 (3 6) (5 7) 1)
    (9 (3 6 7) (5 7) (1 6 7) (1 3 5 6) (3 7) 8 2 4)
    ((3 6 7) 5 (2 3 7) 4 (3 6) (2 3) 1 9 8)
    (4 8 (1 3) (1 3 6) 7 9 2 (3 6) 5)
    ((3 6) 9 (1 2) 5 (1 8) (2 8) 4 (3 6) 7)))

(print-sudoku Unsolvable#7)