(defun new-next-fibo-generator ()
  (let ((x0 0)
	(x1 1))
    (lambda ()
      (let ((next (+ x0 x1)))
	(setf x0 x1)
	(setf x1 next)
	next))))

(defvar *uplimit* 4000000)

(let ((next-fibo (new-next-fibo-generator))
      (sum 0))
  (do ((x (funcall next-fibo) (funcall next-fibo)))
      ((> x *uplimit*) sum)
    (setf sum (if (evenp x)
		  (+ sum x)
		  sum)))
  (format t "~&~S" sum))