anchor.diff:  
     INCF-CL-TESTS> (unzip (pairlis (list 'a 'b 'c) (list 1 2 3)))
     (A B C)
  -  (1 2 3)"
  +  (1 2 3)
  +
  +  INCF-CL-TESTS> (unzip 0)
  +  NIL
  +
  +  INCF-CL-TESTS> (handler-case (floor 1 0)
  +                   (division-by-zero () 'caught-division-by-zero))
  +  CAUGHT-DIVISION-BY-ZERO"
     (is (eq t (doctest :incf-cl-tests))))
  
positive.code: 
  (in-suite test)
  
  (deftest test-drop ()
    (is (eq nil (drop 0 nil)))
    (is (eq nil (drop 1 nil)))
    (is (eq nil (drop -1 (list 1 2 3))))
    (is (equal (list 1 2 3) (drop 0 (list 1 2 3))))
    (is (equal nil (drop 3 (list 1 2 3)))))
  
  (deftest test-drop-while ()
    (is (eq nil (drop-while (constantly t) nil)))
    (is (equal (list 1 2 3) (drop-while (constantly nil)
                                        (list 1 2 3)))))
  
  (deftest test-filter ()
    (is (eq (filter nil nil) nil))
  
negative.code:      (is (equal x (list 1 1 1 3 5)))
       (is (equal y (list 6 8))))))
  
  (deftest test-break ()
    (multiple-value-bind (x y) (break* #'oddp (nreverse (list 1 1 1 3 5 6 8)))
      (is (equal x (list 8 6)))
      (is (equal y (nreverse (list 1 1 1 3 5))))))
  
  (deftest test-split-at ()
    (is (eq (split-at -10 (list 1 2 3)) nil))
    (is (eq (split-at 0 nil) nil))
  