DTD problem

I am writing a simple XML retrieval tool, and I want to be able to find out from the XML documents associated DTD, which elements are allowed under a certain element.

I am using Java, and can retrieve the "model" using JAXP's DeclHandler but it's just a string of text like (it's an example):

(atag|btag*)+ 

is there an available parser for this info?  Having looked at the spec, I don't want to have to write my own if I don't have to!

Paul