If the value cannot be calculated, the attribute should not be shown.

>>>>templates/test.tpl
<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
<opt:root>
<foo>
<opt:attribute name="class">
	<opt:value test="$val1 == 'foo'">a</opt:value>
	<opt:value test="$val1 == 'bar'">b</opt:value>
</opt:attribute>
</foo>
</opt:root>

>>>>data.php
$view->val1 = 'joe';

>>>>expected.txt
OUTPUT

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