Removing hierarchy

I've got an input file like this:
<items>
  <item>
    <a>abc</a>
    <b>def</b>
    <item>
      <a>ghi</a>
      <b>jkl</b>
    </item>
    <item>
      <a>mno</a>
      <b>pqr</b>
      <item>
        <a>stu</a>
        <b>vwx</b>
      </item>
    </item>
  </item
</items

The items can be nested to any depth.

I want to convert this to:

<items>
  <item>
    <level>0</level>
    <a>abc</a>
    <b>def</b>
  </item
  <item>
    <level>1</level>
    <a>ghi</a>
    <b>jkl</b>
  </item>
  <item>
    <level>1</level>
    <a>mno</a>
    <b>pqr</b>
  </item
  <item>
    <level>2</level>
    <a>stu</a>
    <b>vwx</b>
  </item>
</items

In other words I want to remove the hierarchy of items and replace it with
a 'level' element.

What's the best way of doing this?

Many thanks - Rowan


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