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  <para>DocBook documents are SGML documents.  That includes DocBook
   XML documents as well; XML is a subset of SGML.  The SGML
   declaration at the top identifies it as a DocBook document, and it
   should look something like this:</para>

&lt;!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"

  <para>The <sgmltag>book</sgmltag> here specifies the type of the
   top-level element of the document.  This declaration also says that
   the document conforms to version 4.2 of the DocBook XML document
   type declaration (DTD).  The <sgmltag>book</sgmltag> element is also
   shown (although without its content)&mdash;notice that it is opened
   with <quote><literal>&lt;book&gt;</literal></quote> and closed with
   <quote><literal>&lt;/book&gt;</literal></quote>, and everything in
   between is the content of that element.</para>

  <para>Other files can be included by the top-level file by the use of
   entities.  To include a file you first declare an entity and then
   use it:</para>

&lt;!DOCTYPE &hellip;[
&lt;!ENTITY includedfile SYSTEM "includedfile.xml">

  <para>Because SGML tags (<literal>&lt;&hellip;&gt;</literal>) and
   entities (<literal>&amp;&hellip;;</literal>) are interpreted
   specially, there are escapes provided to enable you to use these
   marker characters in your DocBook files.
   <quote><literal>&amp;lt;</literal></quote> gives
   <quote>&lt;</quote>; <quote><literal>&amp;gt;</literal></quote>
   gives <quote>&gt;</quote>; and
   <quote><literal>&amp;amp;</literal></quote> gives
   <quote>&amp;</quote>.  There are many other predefined entities

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