anchor.diff:    (assert-equal '() (rem #'even '(2 12 16 4)))
     (assert-equal '(13 5 7) (rem #'even '(13 5 7)))
     (assert-equal '(3 6 12) (rem 20 '(3 25 6 30 12) :test #'<))
  +  (assert-equal '(1 2 3 5) (rem #'even '(1 2 3 4 5) :start 3))
  +  (assert-equal '(35 3 40 6 12) (rem 20 '(35 3 40 25 6 30 12)
  +                                     :start 3 :test #'<))
     ;; Same tests but with vectors instead.
     (assert-equalp #() (rem 5 #()))
     (assert-equalp #() (rem #'even #()))
  
positive.code:   (assert-equalp #(5 29 5) (rem #'even #(2 5 29 5 28)))
    (assert-equalp #() (rem #'even #(2 12 16 4)))
    (assert-equalp #(13 5 7) (rem #'even #(13 5 7)))
    (assert-equalp #(3 6 12) (rem 20 #(3 25 6 30 12) :test #'<)))
  
  (deftest keep (hof)
    (assert-equal '() (keep 7 '()))
  
negative.code:   (assert-equal '() (keep #'even '(5 7 3)))
    (assert-equal '(2 12 72 6) (keep #'even '(2 12 72 6)))
    (assert-equal '(25 30) (keep 20 '(3 25 6 30 12) :test #'<))
    (assert-equal '(1 2 3 8 2 4) (keep #'even '(1 2 3 8 5 2 4 1) :start 3))
    (assert-equal '(1 25 3 8 2 4) (keep 9 '(1 25 3 30 25 8 2 15 4)
                                        :test #'> :start 3))
    ;; Same tests but for vectors.
    (assert-equalp #() (keep 7 #()))
    (assert-equalp #() (keep #'even #()))
    (assert-equalp #(2 8 2 4) (keep #'even #(1 2 8 2 3 4)))
    (assert-equalp #() (keep #'even #(5 7 3)))
    (assert-equalp #(2 12 72 6) (keep #'even #(2 12 72 6)))
    (assert-equalp #(25 30) (keep 20 #(3 25 6 30 12) :test #'<))
    (assert-equalp #(1 2 3 8 2 4) (keep #'even #(1 2 3 8 5 2 4 1) :start 3))
    (assert-equalp #(1 25 3 8 2 4) (keep 9 #(1 25 3 30 25 8 2 15 4)
                                        :test #'> :start 3)))
  
  ;;; Member does not work on vectors (what is the tail of a vector?).
  (deftest mem (hof)
  