Check if extending snippets work.

>>>>templates/test.tpl
<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
<opt:extend file="content_snippet">
<opt:snippet name="foo">
SNIPPET-VALUE
</opt:snippet>

<opt:snippet name="content_snippet">
BEGIN
<opt:insert snippet="foo"/>
END
</opt:snippet>
</opt:extend>

>>>>expected.txt
OUTPUT

>>>>result.txt
BEGIN
SNIPPET-VALUE
END