Product is Left Identity Therefore Left Cancellable

Theorem
Let $\left({S, \circ}\right)$ be a semigroup.

Let $e_L \in S$ be a left identity of $S$.

Let $a \in S$ such that:
 * $\exists b \in S: b \circ a = e_L$

Then $a$ is left cancellable in $\left({S, \circ}\right)$.