anchor.diff:    (is (eq nil (replicate -1 'x)))
     (is (eq nil (replicate 0 'x)))
     (is (equal (list 1 1 1) (replicate 3 1))))
  +
  +(deftest test-span ()
  +  (let ((xs (list 1 1 1 3 5 6 8)))
  +   (is (eq (span nil xs) nil))
  +   (is (eq (span (constantly t) nil) nil))
  +   (is (eq (span (constantly nil) xs) nil))
  +   (is (equal (span (constantly t) xs) xs))
  +   (multiple-value-bind (x y) (span #'oddp xs)
  +     (is (equal x (list 1 1 1 3 5)))
  +     (is (equal y (list 6 8))))))
  +
  +(deftest test-split-at ()
  +  (is (eq (split-at -10 (list 1 2 3)) nil))
  +  (is (eq (split-at 0 nil) nil))
  +  (is (eq (split-at 0 1) nil))
  +  (multiple-value-bind (x y) (split-at 0 (list 1 2 3))
  +    (is (and (equal x nil)
  +             (equal y (list 1 2 3)))))
  +  (is (equal (split-at 100 (list 1 2 3)) (list 1 2 3)))
  +  (multiple-value-bind (x y) (split-at 1 (list 1 2 3))
  +    (is (and (equal x (list 1))
  +             (equal y (list 2 3))))))
   
   (deftest test-starts-with ()
     (is (eq t (starts-with "Hello, world!" "Hell")))
  
positive.code:   (is (equal (list 1 2 3) (drop-while (constantly nil)
                                        (list 1 2 3)))))
  
  (deftest test-flip ()
    (is (= (funcall (flip #'-) 1 2) 1))
    (is (= (funcall (flip #'-) 2 1) -1)))
  
negative.code:   (is (string= "Hello, world"
                 (string-join (list "Hello" "world") ", "))))
  
  (deftest test-doctest ()
    "This is a sample docstring.
  
  