Permutation of Indices of Supremum

Theorem
Let $\family {a_i}_{i \mathop \in I}$ be a family of elements of the non-negative real numbers $\R_{\ge 0}$ indexed by $I$.

Let $\map R i$ be a propositional functions of $i \in I$.

Let $\ds \sup_{\map R i} a_i$ be the indexed supremum on $\family {a_i}$.

Then:
 * $\ds \sum_{\map R i} a_i = \sum_{\map R {\map \pi i} } a_{\map \pi i}$

where $\pi$ is a permutation on the fiber of truth of $R$.