DTD Subset Reduction Transformation

I have an xml file that validates to a DTD. Let's call the DTD "larger.DTD".

I have another DTD which is a subset of larger.DTD, which we'll call
smaller.DTD.  

I want to strip out a bunch of elements in that xml file, so that it
validates to smaller.DTD.  

In other words, whatever elements/structures in the xml file that the
smaller.DTD doesn't recognize, remove them. (there will probably be some
information lossed of course.) 

For example, transform an xhtml file into a chtml file. (or xhtml Strict to
xhtml Simple).

Is there an automated easy way to do this type of reduction transformation?

(I am using msxml parser, but if you don't use it then please just give me
the theory.)

thanks if you respond,
Mike





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