Hello,

First of all, I would like to thank you for this great and useful project.
There seems to be an issue when serializing a resource, that contains multiple root objects. I've created a very simple metamodel
which contains one class in the top-level package and one in a sub-package: pyecore_multiroot.ecore.txt.
In a test script I create multiple sub-package objects and put them in a resource and save: test.py.txt.
Unfortunately the resulting xmi is broken (the subpackage namespace alias should appear in the top level xmi:XMI node) pyecore.xmi.txt.
I've serialized the same model instance in java (eclipse) and this results in eclipse_java.xmi.txt.

I hope, you find the time to have a look into this issue.

Many greetings
Soeren