[xsl] get attribute value in group element

[henry human <henry_human@xxxxxxxx>]

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?