code: def get_subdomain_entry_at_zonefile_index(self, fqn, zonefile_index, cur=None):
          
          get_cmd = "SELECT * FROM {} WHERE fully_qualified_subdomain=? AND parent_zonefile_index=?".format(self.subdomain_table)
          cursor = None
          if cur is None:
              cursor = self.conn.cursor()
          else:
              cursor = cur
  
          db_query_execute(cursor, get_cmd, (fqn,zonefile_index))
  
          try:
              rowdata = cursor.fetchone()
              assert rowdata
          except Exception as e:
              raise SubdomainNotFound(fqn)
  
          return self._extract_subdomain(rowdata)
docs: Given a fully-qualified subdomain and a sequence number, get its historic subdomain record at that sequence.
          Raises SubdomainNotFound if there is no such subdomain