; Values of expressions can be traced by wrapping them along with a 
; message, which can be anything, as follows:

; Replace any expression giving rise to the value with:
;
;     (show message expression)
;
; The result of this wrapped expression is the same as the value
; of the expression itself there will be a side-effect of displaying
; the value along with the message.
; The display caused by show can be toggled on or off by setting
; the value of variable showbiz to #t or #f.
;
; Once your program is debugged, the wrapped expression can be replaced
; with the original expression.

(define showbiz #f)

(define (show msg value)
  (if showbiz
  (begin
    (display msg)
    (display value)
    (newline)
    value)
  value))