How to assert on condition of another assertion please?

I'm testing a simple medical rule: children of 2 years old should have MMR vaccine done

    1) if a child is not 2 years old, he is not eligible
    2) if a child is 2 years old, he is eligible for vaccine, and if he has done, then he is compliant, otherwise he's not compliant

I'm using data driven testing, the spreadsheet has patients' age and vaccine date (if he's done) like below

Header:      Age      VaccineDate      isEligible       isComplaint

                       2        01/01/2015       true               true

                       1        null                        false              null

                       2        null                        true               false

If a patient is eligible, the xml response returns patientID

If a patient is NOT eligible, the xml response returns nothing

If a patient is eligible, the xml response also returns <compliance>true</compliance>, or <compliance>false</compliance>

For Eligibility I do xPath Assertion, in "xPath Expression" I have

declare namespace NDS='http://....;
exists( //NDS:QualityGuideline/NDS:patientID)

for "Expected Result" I have ${DataSource#isEligible}

So if a patient is eligible, the node of patientID exists, "exists( //NDS:QualityGuideline/NDS:patientID)" is true and it fits "isEligible" (true) in the datasource, if a patient is NOT eligible, the node of patientID doesn't exist, "exists( //NDS:QualityGuideline/NDS:patientID)" is false, it fits "isEligible" (false) in the datasource

For Compliance I do xPath Assertion, in "xPath Expression" I have

declare namespace NDS='http://....;
//NDS:QualityGuideline/NDS:compliance

for "Expected Result" I have ${DataSource#isCompliant}

So if a patient is compliant, I have the response <compliance>true</compliance>, it fits "isCompliant" (true) in the datasource, if a patient is NOT compliant, the response <compliance>false</compliance> fits "isCompliant" (false) in the datasource

The assertion on eligibility works perfectly, but the assertion on compliance only works when a patient is eligible, when a patient is not eligible, there's no response for compliance and it generates an error of "<compliance> doesn't exist"

In order to fix this, I'd like to add a condition for assertion on compliance so it only works when a patient is eligible (which means the node of <patientID> exists in the response), something like:

import com.eviware.soapui.support.XmlHolder

def holder = new XmlHolder( messageExchange.responseContentAsXml )
holder.namespaces["NDS"] = "http://..."
def node = holder.getDomNode( "//NDS:QualityGuideline/patientID")

if node != null        // if the node for <patientID> exists in the response, the patient is eligible, then assert compliance

{//NDS:QualityGuideline/NDS:compliance = ${DataSource#isCompliant} }

Of course the last line doesn't work, because it's xPath assertion not script assertion, so my question is, how do I write assertion on compliance with the condition that it only asserts when patient is eligible (if not eligible it won't assert)

I don't mind using script or xPath assertion or combination, as long as it works

Thanks a lot for help

Jerry