XPATH

Hello,
I'm facing the following problem, it deals with xpath but I really hope I 
can solve it withouth xslt (so maybe this isnt the right place to post it)
I have the following code
<xsd:element name="SUBSTREAM">
   <xsd:complexType>
      <xsd:sequence>
          <xsd:element name="SUBSTREAM_length" type="xsd:short"/>
          <xsd:element name="SUBSTREAM_interior" type="mctf:Sub"/>
      </xsd:sequence>
    </xsd:complexType>
 </xsd:element>


and I want to add the following:

<xsd:simpleType name="Sub">
 <xsd:restriction base="bs1:byteRange">
    <xsd:annotation>
      <xsd:appinfo>
         <bs2:length value="SomeXPathExpressionToSUBSTREAM_length"/>
      </xsd:appinfo>
    </xsd:annotation>
  </xsd:restriction>
</xsd:simpleType>

so i need help with that XPATH expression, cause i think its similar to the 
xpath in xsl no?
greetings Chris

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