[xsl] Intersection of two sets based on a subset of fields?

Hi Folks,

I have a set of Book instances. I call this Set #1. Here are two instances:

<Book>
    <Title>ABC</Title>
    <Author>John Doe</Author>
    <Country>USA</Country>
    <Binding>hardcover</Binding>
</Book>

<Book>
    <Title>XYZ</Title>
    <Author>Mary Sue</Author>
    <Country>CAN</Country>
    <Binding>hardcover</Binding>
</Book>

I have a set of Music instances. I call this Set #2. Here is one instance:

<Music>
    <Genre>Classical</Genre>
    <Author>John Doe</Author>
    <Country>USA</Country>
    <Cost>9.95</Cost>
</Music>

Query: Find the subset of Set #1 that intersects with Set #2 at the <Author> 
and <Country> elements.  

If Set #1 consists of the two instances shown above and Set #2 consists of the 
one instance shown above, then the result of the query is the following:

<Book>
    <Title>ABC</Title>
    <Author>John Doe</Author>
    <Country>USA</Country>
    <Binding>hardcover</Binding>
</Book>

That Book instance has an <Author> and <Country> that intersects with the 
<Author> and <Country> in the Music instance. 

I view this as an intersection problem. However, it is an intersection at 
specific fields (the Author and Country fields). Do you view it as an 
intersection problem?

If it is viewed as an intersection problem, then is there a nice XPath 
expression that does intersection-at-specific-fields?

If it is better viewed not as an intersection problem, then how is it better 
viewed? is there a nice XPath expression for that view?

/Roger 
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