@Override
  public InputSource getExternalSubset(String name, String baseURI)
      throws SAXException, IOException {
    return null;
  }
Given a public id, attempt to resolve it to a DTD. Returns an <code>InputSoure</code> for the
DTD.
geode-core/src/main/java/org/apache/geode/internal/cache/xmlcache/CacheXml.java