1 (define (apply-generic op . args)
2   (let ((type-tags (map type-tag args)))
3     (let ((proc (get op type-tags)))
4       (if proc
5 	  (apply proc (map contents args))
6 	  (error
7 	   "No method for these types -- APPLY-GENERIC"
8 	   (list op type-tags))))))
9 
10 (define (attach-tag type-tag contents)