XSD constraint : highest attribute value in an element

[Stone]

Dear all

If I have the following xml file :

<myxml>
<body>
<text>My text</text>
<num_attach>2</num_attach>
</body>
<file id="1">
user32.dll
</file>
<file id="2">
user.dat
</file>
</myxml>

How can I express (using xml schema) that the value of <num_attach> element must be equal to number of occurences of <file> elements (the highest value of the id) ?

Thanks in advance,
Stone