[xsl] XSLT2/Saxon: How to group by distinct element values?

Hi, even after reading for one hour about grouping in XSLT2, I could not find 
out how to transform my Source XML to my Result XML. 
Can anybody help me?

Source XML
----------
<?xml version="1.0" encoding="UTF-8"?>
<root>
  <data>
    <group1>MERCEDES</group1>
    <group2>CLK</group2>
    <group3>CLK 320 CDI Coupe</group3>
  </data>
  <data>
    <group1>MERCEDES</group1>
    <group2>CLK</group2>
    <group3>CLK 320 CDI Sedan</group3>
  </data>
  <data>
    <group1>MERCEDES</group1>
    <group2>SLK</group2f>
    <group3>SLK Limousine</group3>
  </data>
  <data>
    <group1>AUDI</group1>
    <group2>A2</group2>
    <group3>A2 Coupe</group3>
  </data>
</root>

Result XML 
----------
<?xml version="1.0" encoding="UTF-8"?>
<root>
  <group1 name="MERCEDES">
    <group2 name="CLK">
      <group3 name="CLK 320 CDI Coupe"/>
      <group3 name="CLK 320 CDI Sedan"/>
    </leve2>
    <group2 name="SLK">
      <group3 name="SLK Limousine"/>
    </group>
  </group1>
  <group1 name="AUDI">
    <group2 name="A2">
      <group3 name="A2 Coupe" />
    </group2>
  </group1>
</root>

Thanks in advance
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