Identity of Monoid is Cancellable
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Theorem
Let $\struct {S, \circ}$ be a monoid.
Let $e$ be an identity element of $\struct {S, \circ}$.
Then $e$ is a cancellable element of $\struct {S, \circ}$.
Proof
Let $x, y \in S$ such that $x \circ e = y \circ e$.
Then, by definition of identity element:
- $x = x \circ e = y \circ e = y$
Thus $x = y$.
$\blacksquare$