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