code: def add_statement(self, statement_obj):
          
          if statement_obj.get_id() in self.idx:
              raise ValueError("Statement with id {} already exists!"
                               .format(statement_obj.get_id()))
          self.node.append(statement_obj.get_node())
          self.idx[statement_obj.get_id()] = statement_obj
docs: Adds a statement object to the layer
          @type statement_obj: L{Cstatement}
          @param statement_obj: the statement object