zip() does not seem to work!

The iteration order is a good question. We actually discussed this during the CNERG meeting today and I am considering asking the MOAB mailing list. However, I think the answer is somewhat unimportant. If you have two meshes that do not have the same iteration order, then there is simply no way to combine them. This is an iMesh (and more likely MOAB) restriction, not a PyNE one. That being said, from experience I know that if two meshes are created the same way but with different tags, than the iteration order is indeed the same, which validates the usefulness of these methods.