What's the difference between xdt:anyAtomicType and x

Hi,

I wonder, what is the difference between the xdt:anyAtomicType and 
xs:anySimpleType? It is a type(duh) and hence can code and definitions depend 
on it, but other than that, does it have any "effective" impact?

Why does it exist? If it didn't exist, anySimpleType would have to derive from 
the imaginary "itemType"; is that the reason? 

Can the anyAtomicType be considered a "marker interface" for atomic values, 
but that it in practice is an anySimpleType?

In the XML.com article titled "The XPath 2.0 Data Model"[1] there's a small 
hint:

"The Data Model document adds five new types to the 19 primitive types defined 
in the Part 2 Recommendation: [...] the xdt:anyAtomicType, an abstract type 
that plugs a newly-discovered architectural hole [...]"

What was the architectural hole(or where can I read about it), and has it any 
relation to my question?


Cheers,

		Frans

1.
http://www.xml.com/lpt/a/2005/02/02/xpath2.html