The test checks the opt:single attribute functionality.

>>>>templates/test.tpl
<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
<opt:root>
<foo>
<bar opt:single="yes">
This will not be visible.
</bar>
</foo>
</opt:root>

>>>>expected.txt
OUTPUT

>>>>result.txt
<foo>
<bar />
</foo>