one question regarding the use of Xpath

Dear all,

I have a confusion in the use of Xpath in AttributeSelector Element of XACML

suppose i have a rule "

A Physician is allowed to check the record of Patient X , if an only if he is the Primary care physician of patient X 
now Xpath would b 


/Physician/PhyID = PhysicianID  // I also wanted to check whether he is a valid physician or not.
        AND  
/Physician/patients/patID = patientID of patient X   // for the checking whether Physician is the primary care physician of the Patient X or not.
               

This kind of Xpath is not correct as the 2nd condition can be true for any Physician who is taking care of the Patient X in addition to Primary care Physician
can we introduce some context information like this

 
<Condition> 
<Apply FunctionId="function:string-equal"> 
<Apply FunctionId="function:string-one-and-only"> 
                    <SubjectAttributeDesignator AttributeID="PatientId" DataType="String"/> 
 </Apply> 
  <Apply FunctionId="String-one-and-only-with-subject"> 
<AttributeSelector RequestContextpath="/Hosptial/Physician[phyID='subjectID']/patients/patID/text()"       
                                                                                                                                DateType="String"> 
         </AttributeSelector> 
         </Apply> 
</Apply> 
. 
</Condition> 


is this new funciton additon is possible with XACML. ?? where subject ID represent the caller in this case

May b i am wrong in writing Xpath , if yes plz help .

with Best regards.
Muhammad.