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    <par>
      <switch>
        <audio src="http://smil.nist.gov/interop1/incorrect.wav"
	    system-captions="off" />
	<audio src="http://smil.nist.gov/interop1/correct.wav"
	    system-captions="on" />
      </switch>
    </par>
  </body>
</smil>