It looks like this:

Our invoice database maintains information about invoices submitted by customers, and the parts ordered on those invoices, as well as some summary information about those parts. We'll create our XML structure to hold this information.

Please note that as we're building up our structure over the course of the chapter, some processors may balk at the resultant DTDs created - specifically those that detect orphan element declarations - but the final product should be handled properly by any processor.