private Node parseAdditive() throws ScriptException {
        Node left = parseMultiplicative();
        if (left == null)
            return null;
        Token token;
        while ((token = analyzer.getNextToken()) != null) {
            if (token.getType() != Operators.ADD
                    && token.getType() != Operators.SUB) {
                analyzer.ungetToken(token);
                return left;
            }
            Node right = parseMultiplicative();
            left = new Node(token.getType(), left, right);
        }
        return left;
    }