Creating and using attributes in XSLT - xslt

Question

I'm trying to write xsl that would take something like this:
<Invoices>
<Invoice>
<Customer>
<Type>P</Type>
<First>John</First>
<Last>Doe</Last>
<Id>123-45</Id>
</Customer>
<Amount>1000.00</Amount>
</Invoice>
<Invoice>
<Customer>
<Type>C</Type>
<Name>ACME Inc</Name>
<Id>765-432</Id>
</Customer>
<Amount>3456.78</Amount>
</Invoice>
</Invoices>
and produce something like this:
<Customers>
<Customer type="Person" ref="1">
<FirstName>John</FirstName>
<LastName>Doe</Lastname>
<Id>123-45</Id>
</Customer>
<Customer type="Company" ref="2">
<Name>ACME Inc</Name>
<Id>765-432</Id>
</Customer>
<Customers>
<Invoices>
<Invoice>
<Recipient>1</Recipient>
<Amount>1000.00</Amount>
</Invoice>
<Invoice>
<Recipient>2</Recipient>
<Amount>3456.78</Amount>
</Invoice>
</Invoices>
I.e. the customers should be listed separately with each having an unique ref-attribute and then the invoices should use that attribute to reference the recipient. One of the things causing me problems is that the schema for the output defines the "ref"-attribute having type long, so I can't directly use the value of Id-element, which contains non-numeric characters. I can use e.g. position() to get unique reference, but how do I keep track of the references used?