Are there any tools that do some DTD introspection?

Hi,

I have a problem here that I am not sure if it can be solved with existing 
tools. I am parsing a document with a DOM parser (Xerces, in occurance, but 
it could be any DOM parser). However, when I encounter an error (for 
example an element that is not allowed) I would like to know what elements 
could of been there.

To phrase this generally, I would like a tool that at any point in the 
parsing I can query it to know what elements can start at this point.

Does anybody know of a tool that can perform this?

Thanks!
Daniel Shane