I would like to make a ring over $\mathbb{Q}$ with $n$ variables which are non-commutative and also include their inverses. So I want to generate the free algebra over $\mathbb{Q}$ with generates $x_1, x_2, ..., x_n$ and $x_1^{-1}, x_2^{-1}, ..., x_n^{-1}$. How can I do this?