General Associative Law for Ordinal Sum

Theorem
Let $x$ be a finite ordinal.

Let $\left\langle{a_i}\right\rangle$ be a sequence of ordinals.

Then:


 * $\displaystyle \sum_{i \mathop = 1}^{x + 1} a_i = a_1 + \sum_{i \mathop = 1}^x a_{i + 1}$