(var palindrome? (comp (filter letter?) lower-case (= (reverse %))))

;Arrange
(var calls [])
(function record f
  (fn (var! calls (append f))
      (... (unmocked f) args)))

(mock comp       (record comp)
      filter     (record filter)
      letter?    (record letter?)
      lower-case (record lower-case)
      =          (record =)
      reverse    (record reverse))

(var sentence "In girum imus nocte et consumimur igni.")

;Act
(var result (palindrome? sentence))

(unmock comp filter letter? lower-case = reverse)

;Assert
(assert result)

(var occurred (freqs calls))
(assert (= (len sentence) (occurred letter?)))                  ;letter? is called (len sentence) times
(assert (... = 1 (map occurred [filter lower-case reverse =]))) ;other functions are called only once