/*
 * call-seq:
 *  internal_subset
 *
 * Get the internal subset
 */
static VALUE internal_subset(VALUE self)
{
  xmlNodePtr node;
  xmlDocPtr doc;
  Data_Get_Struct(self, xmlNode, node);

  if(!node->doc) return Qnil;

  doc = node->doc;
  xmlDtdPtr dtd = xmlGetIntSubset(doc);

  if(!dtd) return Qnil;

  return Nokogiri_wrap_xml_node((xmlNodePtr)dtd);
}