May 28th Monday (五月 二十八日 月曜日)

(define (descartes a b)
  (let ((ls '()))
    (for-each (lambda (x)
                (set! ls (append ls (map (lambda (y)
                                   (cons x y)) b)))) a)
    ls))

  The above codes is a implementation how to make a descartes set.