[xsl] Seek an XPath 2.0 expression for checking that each object in a file system has one parent

["Costello, Roger L. costello@xxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

Hi Folks,

I am modeling a file system. Below is a sample instance. D1 means Directory 1,
F1 means File 1, etc. The instance says this: the content of directory 1 is
directory 2 and file 1. The content of directory 2 is file 2. Stated another
way, directory 2 and file 1 are contained in directory 1, and file 2 is
contained in directory 2.

<Root>
    <D1>
        <D2/>
        <F1/>
    </D1>
    <D2>
        <F2/>
    </D2>
</Root>

I want an XPath 2.0 expression which returns true if each object has one
parent. An "object" is a directory or a file. In the example above each object
has one parent, so the XPath should return true. Below is an illegal file
system because F1 has two parents: D1 and D2.

<Root>
    <D1>
        <D2/>
        <F1/>
    </D1>
    <D2>
        <F2/>
        <F1/>
    </D2>
</Root>

The XPath should return false.

This XPath is almost correct:

for $i in /Root/* return for $j in $i/* return not(name($j) =
$i/following-sibling::*/*/name())

I say it is "almost" correct because it returns multiple Booleans, not a
single Boolean result.

Two Questions:

1. What is the correct XPath expression?
2. Is there a different way to model in XML a file system that would enable a
simple XPath expression?

/Roger