/*
 * call-seq:
 *  system_id
 *
 * Get the URI for a SYSTEM or PUBLIC Entity
 */
static VALUE system_id(VALUE self)
{
  xmlEntityPtr node;
  Data_Get_Struct(self, xmlEntity, node);

  if(!node->SystemID) return Qnil;

  return NOKOGIRI_STR_NEW2(node->SystemID);
}