I would like to provide some code in order to use a stack to manage the 
allocation of input and output streams.

It is available in https://gist.github.com/4015700 . Please have a look. 
I think it is more efficient and clearer than the current approach in 
`l3file`. It is obvious that it is fairly rough. And there is still some 
more work to be done in `package` module in order to compatible with 

Best regards,
Qing Lee