1 (define (apply-generic op . args)
2   (define (can-coerce-into? types target-type)
3     (andmap (lambda (type)
4 	      (or (equal? type target-type)
5 		  (get-coercion type target-type)))
6 	    types))
7   (define (find-coercion-target types)
8     (ormap
9      (lambda (target-type)
10        (if (can-coerce-into? types target-type)
11 	   target-type
12 	   #f))
13      types))
14   (define (coerce-all args target-type)
15     (map (lambda (arg)
16 	   (let ((arg-type (type-tag arg)))
17 	     (if (equal? arg-type target-type)
18 		 arg
19 		 ((get-coercion arg-type target-type) arg))))
20 	 args))
21   (define (no-method type-tags)
22     (error "No method for these types"
23 	   (list op type-tags)))
24   (let ((type-tags (map type-tag args)))
25     (let ((proc (get op type-tags)))
26       (if proc
27 	  (apply proc (map contents args))
28 	  (let ((target-type (find-coercion-target type-tags)))
29 	    (if target-type
30 		(apply
31 		 apply-generic
32 		 (append (list op)
33 			 (coerce-all args target-type)))
34 		(no-method type-tags)))))))