Use
grade.xml and grade.xsl in ~cs301/public_html/xml/example as an example.
Use purchase.xml in hw3/4 as a template to specify the Document Type Data
file, called it purchase.dtd, for checking the other purchase documents
following the same document structure. Save purchase.xml and purchase.dtd
in the xml directory.
Add the following line as the third line in purchase.xml, right after
the two processing instruction line.
<!DOCTYPE purchase SYSTEM "purchase.dtd">