WorkflowTools - Rollup fuctions - fetch XML retrive PARENT_GUID

Hi

We are trying to create a Rollup Function form workflowtools as described in this post: github.com/.../Rollup%20Functions.md

The result will be a function that rolls up a value fields on opportunity child records and posts it on the opportunity parent record. 

We use the following query: 

<fetch version="1.0" output-format="xml-platform" mapping="logical" distinct="false">
<entity name="ncg_productline">
<attribute name="ncg_value" />
<attribute name="ncg_productlineid" />
<order descending="false" attribute="ncg_value" />
<filter type="and">
<condition attribute="ncg_opportunityid" operator="eq" uitype="opportunity" value="{PARENT_GUID}" />
</filter>
</entity>
</fetch>

The issue seems to be with the PARENT_GUID not retriving the parent record. If we put in a static guid then everything works fine. 

Does anyone have an idea to how we can retrive a dynamics parent guid in the query?

Thanks for help