shouldn't xesext.xsd allow for nested attributes?

Hi all,

I am wondering why the xesext.xsd does not permit xesextensions with nested attributes, like this:

<?xml version="1.0" encoding="utf-8"?>
<xesextension name="Test" prefix="test" uri="file://D:/test.xesext">
  <trace>
    <string key="element">
      <alias mapping="EN" name="an element"/>
      <string key="type">
        <alias mapping="EN" name="the type"/>
      </string>
      <float key="amount">
        <alias mapping="EN" name="an amount"/>
      </float>
    </string>
  </trace>
</xesextension>

Has anyone else had the need to create such extensions or have an explanation or know how it can be done?