Beginner query

Hi,

I hope I'm mailing the correct place. 

I am wanting to permit a request, provided certain attributes are
present. I don't care about their value, just the fact that they are
present. I cannot see any way of doing this without implementing my own
match function. Is this correct or have I missed something? I believe my
final policy doc would look like this:

<?xml version="1.0" encoding="UTF-8"?>
<Policy xmlns="urn:oasis:names:tc:xacml:1.0:policy"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

xsi:schemaLocation="http://www.oasis-open.org/tc/xacml/1.0/cs-xacml-schema=policy-01.xsd"

RuleCombiningAlgId="urn:oasis:names:tc:xacml:1.0:rule-combining-algorithm:permit-overrides">
        
	<Description>Policy template for attribute checking</Description>
        
    <!-- This policy document applies to all requests -->      
    <Target>
      <Subjects>
        <AnySubject/>
      </Subjects>
      <Resources>
        <AnyResource/>
      </Resources>
      <Actions>
        <AnyAction/>
      </Actions>
    </Target>      
          
    <!-- Rules for attributes -->      
    <Rule RuleId="EmailRule" Effect="Permit">
      <Description>Allow Email address</Description>
      <Target>
        <Subjects>
          <Subject>
            <ResourceMatch MatchId="isPresent">
              <AttributeValue
DataType="http://www.w3.org/2001/XMLSchema#string"></AttributeValue>
              <SubjectAttributeDesignator
DataType="http://www.w3.org/2001/XMLSchema#anyURI"
                                         AttributeId="emailAddress"/>
            </SubjectMatch>
          </Subject>
        </Subjects>
        <Resources>
          <AnyResource/>
        </Resources>
        <Actions>
          <AnyAction/>
        </Actions>
      </Target>
    </Rule>
 </Policy>

Thanks
P.