def add_statement(self, statement_obj):
        """
        Adds a statement object to the layer
        @type statement_obj: L{Cstatement}
        @param statement_obj: the statement object
        """
        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
Adds a statement object to the layer
        @type statement_obj: L{Cstatement}
        @param statement_obj: the statement object