Proof: Conformity
(related to Proposition: Uniqueness of Inverse Elements)
 By hypothesis, $x\in X$ has at least one inverse element $y\in X.$
 Let $z\in X$ be another inverse element of $x.$
 By definition of inverse elements, we have $x\ast y=y\ast x=e$ and $x\ast z=z\ast x=e,$ where $e$ is the unique neutral element.
 Since "$\ast$" is an associative, it follows $z=z\ast e=z\ast(x\ast y)=(z\ast x)\ast y=e\ast y=y.$
 Therefore, $y$ is unique.
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References
Bibliography
 Fischer, Gerd: "Lehrbuch der Algebra", Springer Spektrum, 2017, 4th Edition