<!-- optional fields for an operating system -->
<optional><element name="distro"><text/></element></optional>
<!-- optional fields for an operating system -->
<optional><element name="distro"><text/></element></optional>
<optional><element name="major_version"><text/></element></optional>
<optional><element name="minor_version"><text/></element></optional>
<optional><element name="package_format"><text/></element></optional>
<optional><element name="major_version"><text/></element></optional>
<optional><element name="minor_version"><text/></element></optional>
<optional><element name="package_format"><text/></element></optional>