code: public void onDocumentType (@Nonnull final String sQualifiedElementName,
                                @Nullable final String sPublicID,
                                @Nullable final String sSystemID)
    {
      ValueEnforcer.notNull (sQualifiedElementName, "QualifiedElementName");
  
      final String sDocType = getDocTypeXMLRepresentation (m_eXMLVersion,
                                                           m_aXMLWriterSettings.getIncorrectCharacterHandling (),
                                                           sQualifiedElementName,
                                                           sPublicID,
                                                           sSystemID);
      _append (sDocType);
      newLine ();
    }
docs: On XML document type.
  
  @param sQualifiedElementName
  Qualified name of the root element.
  @param sPublicID
  Document type public ID
  @param sSystemID
  Document type system ID