you need to write (and reference) a DTD in the header of the xml that corresponds to the containers that you are using.
Tough to explain, but, browse this and you should be able to figure out 99% of what you need.
http://www.w3schools.com/dtd/
I would think you need to just define the containers that you are using. Now you see why everyone else bastardizes their xml to use the standard DTD.
