Generalized Sum is Monotone

Theorem
Let $\left({a_i}\right)_{i \in I}$ be an $I$-indexed family of positive real numbers.

That is, let $a_i \in \R_{\ge 0}$ for all $i \in I$.

Then, for every finite subset $F$ of $I$:


 * $\displaystyle \sum_{i \mathop \in F} a_i \le \sum \left\{{a_i : i \in I}\right\}$

provided the sum on the right converges.