Associative Law of Addition

Theorem
Let $\mathbb F$ be one of the standard number sets: $\N, \Z, \Q, \R$ and $\C$.

Then:
 * $\forall x, y, z \in \mathbb F: x + \paren {y + z} = \paren {x + y} + z$

That is, the operation of addition on the standard number sets is associative.