The smallest possible presentation looks like this:

<?xml version='1.0'?>
<!DOCTYPE slides PUBLIC "-//Norman Walsh//DTD Slides XML V3.0b1//EN"
                 "http://docbook.sourceforge.net/release/slides/3.0b1/slides.dtd">
<slides>
<slidesinfo>
<title>Presentation Title</title>
</slidesinfo>
<foil><title>Foil Title</title>
<para>Foil content</para>
</foil>
</slides>

Every presentation must contain at least one slide.