Parity Addition is Associative/Proof 2

Proof
Let $a, b, c \in R$.

That is, $a, b, c$ are all either $\text{even}$ or $\text{odd}$.

By definition of odd:
 * $\text{odd} = 2 m + 1$

for some $m \in \Z$.

By definition of even:
 * $\text{even} = 2 n + 0$

for some $n \in \Z$.

Thus we can define the mapping $f: R \to \Z$ as:
 * $\forall x \in R: \map f x := \begin{cases}

0 & : x \text { is even} \\ 1 & : x \text { is odd} \end{cases}$

Thus an element of $R$ can be expressed as an arbitrary integer of the form:
 * $x = 2 k + \map f x$

where:
 * $k \in \Z$ is an integer
 * $\map f x$ is either $0$ or $1$ according to whether $x$ is even or odd.

Let $+_2$ be used to denote the operation of $+$ in $R$, that is, the addition of two parities.

Then:

The result follows by the identification of $+_2$ to be used to denote the operation of $+$ in $R$:
 * $\paren {a + b} + c = a + \paren {b + c}$