; Return half the length of the list L.
(define (half-length L)
  (truncate (/ (length L) 2)) )

; Determine if x is a possible median of the values list,
; a list of numbers. It is assumed that x is an element of values.
(define (possible-median? x values)
  (= (how-many-< x values) (half-length values)) )

; Find the median of the values list, a list of numbers.
; *** NOTE: This function does not work correctly when values
; contains duplicate numbers. ***
(define (median values)
  (find-if
    (lambda (x)
      (possible-median? x values))
    values))

; Return the list of numbers in the values list that are smaller than x.
; x is a number, values is a list of numbers.
(define (smaller-vals x values)
  (keep-if (lambda (y) (< y x)) values))

; Return how many numbers in the values list are smaller than x.
; x is a number, values is a list of numbers.
(define (how-many-< x values)
  (length (smaller-vals x values)) )