Or, Using Attribute Syntax
<?xml version="1.0"?>
<COURSE xmlns:xlink="http://www.w3.org/XML/XLink/0.9"
xlink:type="group">
<CLASS xlink:type="document" xlink:href="week1/index.xml"/>
<CLASS xlink:type="document" xlink:href="week2/index.xml"/>
<CLASS xlink:type="document" xlink:href="week3/index.xml"/>
<CLASS xlink:type="document" xlink:href="week4/index.xml"/>
<CLASS xlink:type="document" xlink:href="week5/index.xml"/>
<CLASS xlink:type="document" xlink:href="week6/index.xml"/>
<CLASS xlink:type="document" xlink:href="week7/index.xml"/>
<CLASS xlink:type="document" xlink:href="week8/index.xml"/>
<CLASS xlink:type="document" xlink:href="week9/index.xml"/>
<CLASS xlink:type="document" xlink:href="week10/index.xml"/>
<CLASS xlink:type="document" xlink:href="week11/index.xml"/>
<CLASS xlink:type="document" xlink:href="week12/index.xml"/>
<CLASS xlink:type="document" xlink:href="week13/index.xml"/>
</COURSE>