; Return the square of the number x.
(define (square x)
  (* x x) )

; Return the list of squares of values, a list of numbers.
(define (squared values)
  (map square values) )

; Return the standard deviation for the numbers in values,
; a list of numbers.
(define (std-dev values)
  (sqrt
    (- (mean (squared values)) (square (mean values))) ) )