(define (duple n x)
  (cond ((= n 0) '())
        (else (cons x (duple (- n 1) x)))))


; certification?

; it will be convenient to have a standard form for presenting inductive
; data definitions.  

; likely the following (BNF form) is familiar to you from prior courses


;  <list-of-numbers> ::=  () | (number> . <list-of-numbers>


; another example is the BNF grammar for slists:


; <s-list> ::= ()
;          ::= <symbol-expression> . <s-list>
; 
; <symbol-expression> ::= <symbol> | <s-list>

; some examples:






; we try as much as possible to exploit inductive definitions of data in developing
; programs which manipulate this data.  

; for example: 

; we want to write a program subst which takes three arguments -
; the symbols new and old, and an s-list, slist.  All elements
; of slist are examined, and a new list is returned that is similar
; to slist but with all occurrences of old replaced by instances
; of new

; some examples:


(define subst
  (lambda (new old slist)
    (if (null? slist)
	'()
	(cons 
         (subst-in-symbol-expression new old (car slist))
         (subst new old (cdr slist))))))


(define subst-in-symbol-expression
  (lambda (new old se)
    (if (symbol? se)
	(if (eq? se old) new se)
	(subst new old se))))



; As you can see, the program doesn't work on the list containing numbers.
; If you dig deeper, you can find that is because that the scheme distinguish
; the numbers from symbols.  

; Homework

; Change the subst program to make it work even there are some numbers in the  
; list
;