Set of Integer Combinations includes those Integers

Lemma
Let $a, b \in \Z$ be integers.

Let $S = \set {a x + b y: x, y \in \Z}$ be the set of integer combinations of $a$ and $b$.

Then $a \in S$ and $b \in S$.

Proof
By setting $x = 1$ and $y = 0$:
 * $a \cdot 1 + b \cdot 0 = a$

By setting $x = 0$ and $y = 1$:
 * $a \cdot 0 + b \cdot 1 = b$