> I have a few questions:
> 
> 1. An XPath expression operates on one or more in-memory node trees.
> 
>    (a) True
> 
> 2. An XPath expression cannot change an in-memory node tree.  
> (More precisely, "Evaluating an XPath expression cannot 
> result in changes to any in-memory node trees.")
> 
>    (a) True
> 
> 3. XPath can only be used to:
>      - navigate through an in-memory node tree
>      - retrieve values from the in-memory node tree
>      - operate on the values it retrieves (the result of an 
> operation does not modify the in-memory node tree)
> 
>    (a) True
> 
> 4. An XSLT element operates on one or more in-memory node trees.
> 
>    (a) True
> 
> 5. An XSLT element can change an in-memory node tree.  (More 
> precisely, "Evaluating an XSLT element can result in changes 
> to one or more in-memory node trees.")
> 
>    (b) False - XSLT can create new trees but it cannot modify existing
trees.

> 
> 6. The "in-memory node tree" referenced above is always a "DOM tree."
> Thus, for example, (1) is more precisely phrased as: An XPath 
> expression operates on one or more DOM trees.

>    (b) False - the data model is XDM, which is subtly different from DOM
in a number of important ways. For example, namespaces are modelled quite
differently.
> 

Michael Kay
http://www.saxonica.com/