title: Internal Subset DTD referencing External DTD
body: <p>I have a number of XML documents that use the same main Document Type Definition (mrinitialman.dtd).  Certain elements get a small tweak depending on which page they are in: for example, the SetPage element, which is defined so:</p>
  <pre><code>&lt;!ELEMENT SetPage ((SetImgInf|SetRecInf)?, SetPageName,SetPageDesc?,%credits;)&gt;
              
              &lt;!ELEMENT SetPageName (#PCDATA)&gt;
              &lt;!ELEMENT SetPageDesc (#PCDATA)&gt;
  </code></pre>
  <p>SetImgInf and SetRecInf are not defined in this document type definition; they are defined in the doctype subsets of the files in which those elements are needed.</p>
  <p>For example, recordings.xml has this for a Document Type Declaration:</p>
  <pre><code>&lt;!DOCTYPE MainPage SYSTEM &quot;../Site_Data/XML_Etc/mrinitialman.dtd&quot;[
      &lt;!ELEMENT SetRecInf EMPTY&gt;
      &lt;!ATTLIST SetRecInf recfile CDATA #REQUIRED&gt;
  ]&gt;
  </code></pre>
  <p>I define SetRecInf there because it is the <em>only</em> file to use that element.</p>
  <p>Two files, however, use SetImgInf: art_gallery.xml and photo_gallery.xml; their doctypes are so:</p>
  <pre><code>&lt;!DOCTYPE MainPage SYSTEM &quot;../Site_Data/XML_Etc/mrinitialman.dtd&quot; [
      &lt;!ELEMENT SetImgInf EMPTY&gt;
          &lt;!ATTLIST SetImgInf
              imgkeywords CDATA   #IMPLIED
              imgfile     NMTOKEN #REQUIRED
              imgformat (jpg|gif|png|tff|bmp) #IMPLIED 
              thumbformat (jpg|gif|png|tff|bmp) #IMPLIED
          &gt;
  
  ]&gt;
  </code></pre>
  <p>Is it possible to put that bit into its own DTD, and thus have both art_gallery.xml and photo_gallery.xml to effectively reference two separate external document type definitions?</p>