EDitorial suggestion for section 1.2

Secction 1.2 does not mention the fallback element as a difference 
between external entities and XInclude. I think this is a crucial and 
particularly easy to understand difference. I would add a paragraph 
mentioning this, perhaps something like this:

Failure to load an external entity is normally a fatal error. 
XInclude allows the author to provide default content that will be 
used if the remote resource cannot be loaded.
-- 

+-----------------------+------------------------+-------------------+
| Elliotte Rusty Harold | elharo@metalab.unc.edu | Writer/Programmer |
+-----------------------+------------------------+-------------------+
|          The XML Bible, 2nd Edition (Hungry Minds, 2001)           |
|             http://www.cafeconleche.org/books/bible2/              |
|   http://www.amazon.com/exec/obidos/ISBN=0764547607/cafeaulaitA/   |
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