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HTML is good for debugging, but probably is not suitable for your application.
See complete documentation, or API help for more information. 
<?xml version="1.0"?>
<api>
  <query>
    <allusers>
      <u name="Yf" />
      <u name="Yin" />
      <u name="Zagari" />
      <u name="Zagari 2.0" />
      <u name="Zagarius" />
      <u name="Zeta1127,89thLegion" />
      <u name="Zf" />
      <u name="ZoeHild" />
    </allusers>
  </query>
</api>