gistfile1.scm

(define-rewriter do 
  (lambda (form rename)
    (let ((bindings (map (lambda (x) (cons (car x) (list (cadr x)))) (cadr form)))
          (steps (map (lambda (x) (if (< 2 (length x)) (caddr x) (car x))) (cadr form)))
          (condition (car (car (cddr form))))
          (endresult (cadr (car (cddr form))))
          (expr (cdddr form))
          (loop-name (gensym)))

      `(,(rename 'let) ,loop-name ,bindings
        (,(rename 'if) (,(rename 'not) ,condition)
            (,(rename 'begin) ,@expr ,(cons loop-name steps))
	    ,endresult)))))