(defun Y (f)
  (funcall
    (lambda (g) (funcall g g))
    (lambda (h)
       (funcall f (lambda args (apply (funcall h h) args))))))
(defun fibo-impl (self)
  (lambda (n)
    (if (<= n 2)
        1
        (+ (funcall self (- n 1)) (funcall self (- n 2))))))
(defvar fibo (Y #'fibo-impl))
(assert (= 55 (funcall fibo 10)))

(defun generate-counter (init)
  (let ((i init))
    (lambda ()
      (setq i (+ 1 i))
      i)))
(let ((c (generate-counter 0)))
  (assert (= 1 (funcall c)))
  (assert (= 2 (funcall c)))
  (assert (= 3 (funcall c))))

(let ((x 1)
      (fn nil))
  (setq fn
    (lambda (x) (setq x 2)))
  (funcall fn x)
  (assert (= x 1)))

(assert-error
  (let
    ((f
       (lambda (x) (funcall f 1) x)))
    (funcall f 1)))

(defun my-add (lambda (x y) (+ x y)))
(assert (= 3 (my-add 1 2)))

(defun my-add #'+)
(assert (= 3 (my-add 1 2)))

(defvar flag 0)
(defun func ()
  (incq flag)
  (incq flag))
(defun func
  (let ((old-func #'func))
    (lambda ()
      (funcall old-func)
      (incq flag))))
(func)
(assert (= 3 flag))