Inverse of Inverse/General Algebraic Structure

Theorem
Let $\struct {S, \circ}$ be an algebraic structure with an identity element $e$.

Let $x \in S$ be invertible, and let $y$ be an inverse of $x$.

Then $x$ is also an inverse of $y$.

Proof
Let $x \in S$ be invertible, where $y$ is an inverse of $x$.

Then:
 * $x \circ y = e = y \circ x$

by definition.