[xsl] get attribute value in group element

Hi
Without using for-each I want to get from the following xml the value of the 
num3 attribute(the value is 33) , if the num1 attribute has a value of 13.
<db>
  <group>
  <name>Team1</name>
    <set num1="11" num2="21" num3="31"></set>
      <set num1="12" num2="22" num3="32"></set>
      <set num1="13" num2="23" num3="33"></set>
  </group>  
  <otherGroup>
...
  </otherGroup>
</db>
Something like 
<xsl:value-of select= "document('db.xml')/db/group/set[@='num1']   ??????   "/>
Any hint?

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