Difference in Naturally Ordered Semigroup is Unique

Theorem
Let $\struct {S, \circ, \preceq}$ be a naturally ordered semigroup.

Let $n, m \in S$ such that $m \preceq n$.

Then there exists a unique difference $n - m$ of $m$ and $n$.

Proof
Since $m \preceq n$, by :


 * $\exists p \in S: m + p = n$

Now suppose that $p, q \in S$ are such that:


 * $m + p = m + q = n$

Then it follows from that:


 * $p = q$

Hence the result.