creating XML hierarchy where not existed before??

Hi,

I've been banging my head on this for a while now: I
need to create hierarchical XML out of flat XML. The
logic is simple, but I think that I can't perform this
translation in XSL because I can't proceed through the
input file ina serial manner.

In the example that follows:
all group3s following a group2 must be included in the
last group2; all group2s follwing a group1 must be
included in the last group1. There can be any number
of groups that I would like to nest.

Can this problem be solved using XSLT, or will I have
to look somewhere else to process my input files?

Any help greatly appreciated,

//F

INPUT:
<group1>
  <datafield1>value1</datafield1>
  <datafield2>value2</datafield2>
</group1>

<group2>
  <datafield1>value1</datafield1>
  <datafield2>value2</datafield2>
</group2>

<group2>
  <datafield1>another_value1</datafield1>
  <datafield2>another_value2</datafield2>
</group2>

<group3>
  <datafield1>value1</datafield1>
  <datafield2>value2</datafield2>
</group3>

Desired Output:

<group1>
  <datafield1>value1</datafield1>
  <datafield2>value2</datafield2>

  <group2>
    <datafield1>value1</datafield1>
    <datafield2>value2</datafield2>
  </group2>

  <group2>
    <datafield1>another_value1</datafield1>
    <datafield2>another_value2</datafield2>

    <group3>
      <datafield1>value1</datafield1>
      <datafield2>value2</datafield2>
    </group3>

   </group2>

</group1>

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