XForm/XPath Calculate=""???

[kent_will]

<bind id="cRequiredVA" nodeset="RequiredVA" calculate="sum(Notes/Unit/VARating)" type="string"/>

I'm using the above bind and I know that its wrong but what I want to do is...

<bind id="cRequiredVA" nodeset="RequiredVA" calculate="sum(Notes/Unit/VARating * Notes/Unit/@qty)" type="string"/>

Basically for each Unit the VA * Qty, and I want the sum of all the results.

I've written an XSL template for this kind of reccursive multiplication...can I call templates from an XForm bind??