[plt-scheme] module hierarchy

Hi,
I like the protection that modules provide against modifying their variables 
by code that requires the modules. I also like the shielding of scope, even 
in syntax definitions. Beautiful! A module may, in thought, be devided in 
submodules, the main module simply requiring the submodules and calling or 
providing whatever is to be supplied to the user. Likewise submodules may 
require subsubmodules, and so on. However, as I understand, every module 
must be located in its own file, which makes all submodules evenly 
accessible to the user, contrary to my concept of dividing a main module in 
hidden submodules. Is there a technical reason why a module must not contain 
other modules? If there is no such technical problem, has it been considered 
to allow submodules to be declared and required within a module?
Jos Koot
((((lambda(x)((((((x x)x)x)x)x)x))
   (lambda(x)(lambda(y)(x(x y)))))
  (lambda(x)(write x)x))
 "greetings, Jos")