anchor.diff:      (return-from outer 'valve)))
   
   (define-test test-code-block-01
  -  (assert-equal ___ (code-block-01)))
  +  (assert-equal 4 (code-block-01)))
   
   (define-test test-code-block-02
  -  (assert-equal ___ (code-block-02)))
  +  (assert-equal 'space (code-block-02)))
   
   
   ;; About closures and the distinction of lexical and dynamic bindings
  
positive.code: ;; then invoke f to see which value of x is returned.
  
  (define-test test-lexical-bindings-may-be-shadowed
    (assert-eq ___ (let ((f (let ((x 10))
                   (lambda () x))))  ; <-- x bound lexically
      (let ((x 20))          ; form 2
        (funcall f)))))
  
  
  (define-test test-special-bindings-look-back-on-execution-path
    (assert-eq ___ (let ((f (let ((x 10))
               (declare (special x))
               (lambda () x))))      ; <-- x bound dynamically
      (let ((x 20))          ; form 2
  
negative.code:               nil)))
  
  (define-test test-shadowing-a-variable
    (assert-equal '(1 2) (shadow-z 1)))
  
  
  (defun code-block-01 ()
  