GOOPS, generics and module system

[Нуёпт]

Please tell me what's wrong with this code:

;;; first program

(define-module (my-module)
        #:use-module (oop goops)
        #:use-module (module a)
        #:use-module (module b)
        #:duplicates merge-generics
)

(display (inside? (make <a>)))(newline)
(display (inside? (make <b>)))(newline)

;;; module a

(define-module (module a)
        #:use-module (oop goops)
)

(define-class <a> () x)

(define-method (inside? (a <a>)) "class a" )

(export inside?)
(export <a>)

;;; module b

(define-module (module b)
        #:use-module (oop goops)
)

(define-class <b> () x y )

(define-method (inside? (b <b>)) "class b" )

;(export inside?)
(export <b>)

Such error occurred if `(export inside?)' is uncommented:
ERROR: Unbound variable: inside?

Else:
class a
ERROR: No applicable method for #<<generic> inside? (1)> in call (inside? #<<b> 
94afdc0>)

I want that first program can include modules a and b, second include
a, n and p, for example, and so on, i.e. modules a, b, c... must not know about 
each other, but
can provide generic methods.