                            Some(right) => Some(Expr::arithmetic_op(right, new_op.unwrap(), expr.clone().unwrap())),
                            None => expr
                        };
                    },
                    _ => {
                        self.drop_lexem();

                        return match right {
                            Some(right) => Ok(Some(Expr::arithmetic_op(left.unwrap(), op.unwrap(), right))),
                            None => Ok(left)
                        }