schematron, multiple elements values compare - schematron

Question

In schematron I have 2 files. In source01.xml there is a keyvalue (xacml-context:AttributeValue), with this keyvalue I want get the minimum value of servicelevel in source02.xml by comparing the value of the keyvalue with the value of "eh:ServiceProviderID" in source02.xml.
In de following example I want get "2" as result.
source01.xml
<xacml-context:Resource>
<xacml-context:Attribute AttributeId="urn:etoegang:core:ServiceID" DataType="http://www.w3.org/2001/XMLSchema#string">
<xacml-context:AttributeValue>01002</xacml-context:AttributeValue>
</xacml-context:Attribute>
<xacml-context:Attribute AttributeId="urn:etoegang:core:ServiceUUID" DataType="http://www.w3.org/2001/XMLSchema#string">
<xacml-context:AttributeValue>
</xacml-context:AttributeValue>
source02.xml
<eh:ServiceProvider>
<eh:ServiceProviderID>01001</eh:ServiceProviderID>
<eh:OrganizationDisplayName>company A</eh:OrganizationDisplayName>
<eh:Service>
<eh:ServiceID>DV:01506</eh:ServiceID>
<eh:ServiceLevel>1</eh:ServiceID>
...
</eh:Service>
<eh:Service>
<eh:ServiceID>DV:02506</eh:ServiceID>
<eh:ServiceLevel>2</eh:ServiceID>
...
</eh:Service>
<eh:Service>
<eh:ServiceID>DV:03506</eh:ServiceID>
<eh:ServiceLevel>3</eh:ServiceID>
...
</eh:Service>
</eh:ServiceProvider>
<eh:ServiceProvider>
<eh:ServiceProviderID>01002</eh:ServiceProviderID>
<eh:OrganizationDisplayName>company B</eh:OrganizationDisplayName>
<eh:Service>
<eh:ServiceID>DV:01507</eh:ServiceID>
<eh:ServiceLevel>2</eh:ServiceID>
...
</eh:Service>
<eh:Service>
<eh:ServiceID>DV:02507</eh:ServiceID>
<eh:ServiceLevel>2+</eh:ServiceID>
...
</eh:Service>
<eh:Service>
<eh:ServiceID>DV:03507</eh:ServiceID>
<eh:ServiceLevel>3</eh:ServiceID>
...
</eh:Service>
</eh:ServiceProvider>
<eh:ServiceProvider>
<eh:ServiceProviderID>01003</eh:ServiceProviderID>
<eh:OrganizationDisplayName>company C</eh:OrganizationDisplayName>
<eh:Service>
<eh:ServiceID>DV:02508</eh:ServiceID>
<eh:ServiceLevel>1</eh:ServiceID>
...
</eh:Service>
<eh:Service>
<eh:ServiceID>DV:03508</eh:ServiceID>
<eh:ServiceLevel>3</eh:ServiceID>
...
</eh:Service>
</eh:ServiceProvider>
schematron
<rule context="xacml-c:Attribute[#AttributeId='urn:etoegang:core:ServiceID']">
<let name="ServiceIdValue" value="xacml-c:AttributeValue"/>
<assert test="$ServiceIdValue=' '">ServiceID (<value-of select="$ServiceIdValue"/>) is the keyvalue.</assert>
<let name="DCValue" value="document('source02.xml')//*[name()='eh:ServiceProviderID']"/>
<assert test="$DCValue=' '">DCValue (<value-of select="$DCValue"/>) is the source02 value. </assert>
</rule>