[xsl] maintaining sequence numbers over sessions

[fred@xxxxxxxxxxxxx]

Hi,

My app (or script) is entirely client based. No server involved.
It produces forms (from schemas) and the result XML is stored locally.

Now, I do not want that file are overridden, so I want to name the files
Invoice-001.xml
Invoice-002.xml
etc.

The sequence number may be assigned when the form (HTML) code is produced.

Is there any way to keep a sequence number (per document type)  
somewhere and update it using XSLT?

Thanks,

Fred