(define (type-tag z)
  (cond ((pair? z) (car z))
        ((number? z) 'scheme-number)
        (else 
          (error "type error"))))

(define (contents z)
  (cond ((number? z) z)
        ((pair? z) (cdr z))
        (else 
          (error "type error"))))

(define (attach-tag type-tag content)
  (if (number? content)
      content
      (cons type-tag content)))