"opt:root" should not be used in other context than parent's.

>>>>templates/test.tpl
<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
<div>
<opt:root>foo</opt:root>
</div>

>>>>data.php
$this->tpl->singleRootNode = true;

>>>>expected.txt
Opt_InstructionInvalidParent_Exception