Left Identity Element is Idempotent

Theorem
Let $\left({S, \circ}\right)$ be an algebraic structure.

Let $e_L \in S$ be a left identity with respect to $\circ$.

Then $e_L$ is idempotent under $\circ$.

Proof
By the definition of a left identity:
 * $\forall x \in S: e_L \circ x = x$

Thus in particular:
 * $e_L \circ e_L = e_L$

Therefore $e_L$ is idempotent under $\circ$.