Ignorable Whitespace in Fragments

I'm implementing XUpdate for a binary file format. I'd like to
    be able to determine which text nodes in an inserted document
    fragment ignoreable whitespace, so I can omit those nodes from
    the file.
    
    I've not worked with XMLSchema, or DTDs, except to get pass/fail
    validation on a full document. I'm wondering how to check the
    validity of a specific node, most importantly, if the a
    whitespace text node is meainful.

    Hope I asked the question somewhat correctly.

    Cheers.

--
Alan Gutierrez - 
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