According to bug #86, the comments placed after an expression cause it not to
be executed.

>>>>templates/test.tpl
<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
<opt:root>
{$foo is true}
<!-- A comment -->

<b opt:if="$foo === true">IT WORKS</b>
</opt:root>

>>>>expected.txt
OUTPUT

>>>>result.txt
<b>IT WORKS</b>