&lt;!DOCTYPE para PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
          "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd"&gt;
&lt;para&gt;
The equation e&lt;superscript&gt;&amp;pi;i&lt;/superscript&gt; + 1 = 0 ties together
five of the most important mathematical constants.
&lt;/para&gt;