1 (define (attach-tag type-tag contents)
2   (if (= type-tag 'scheme-number)
3       contents
4       (cons type-tag contents)))
5 (define (type-tag datum)
6   (cond ((number? datum) 'scheme-number)
7 	((pair? datum) (car datum))
8 	(else (error "Bad tagged datum -- TYPE-TAG" datum))))
9 (define (contents datum)
10   (cond ((number? datum) datum)
11 	((pair? datum) (cdr datum))
12 	(else (error "Bad tagged datum -- CONTENTS" datum))))