/*
 * call-seq:
 *  create_internal_subset(name, external_id, system_id)
 *
 * Create the internal subset of a document.
 *
 *   doc.create_internal_subset("chapter", "-//OASIS//DTD DocBook XML//EN", "chapter.dtd")
 *   # => <!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML//EN" "chapter.dtd">
 *
 *   doc.create_internal_subset("chapter", nil, "chapter.dtd")
 *   # => <!DOCTYPE chapter SYSTEM "chapter.dtd">
 */
static VALUE create_internal_subset(VALUE self, VALUE name, VALUE external_id, VALUE system_id)
{
  xmlNodePtr node;
  xmlDocPtr doc;
  xmlDtdPtr dtd;

  Data_Get_Struct(self, xmlNode, node);

  doc = node->doc;

  if(xmlGetIntSubset(doc))
    rb_raise(rb_eRuntimeError, "Document already has an internal subset");

  dtd = xmlCreateIntSubset(
      doc,
      NIL_P(name)        ? NULL : (const xmlChar *)StringValuePtr(name),
      NIL_P(external_id) ? NULL : (const xmlChar *)StringValuePtr(external_id),
      NIL_P(system_id)   ? NULL : (const xmlChar *)StringValuePtr(system_id)
  );

  if(!dtd) return Qnil;

  return Nokogiri_wrap_xml_node(Qnil, (xmlNodePtr)dtd);
}