Partition of Indexing Set induces Bijection on Family of Sets

Theorem
Let $I$ be an indexing set.

Let $\family {S_\alpha}_{\alpha \mathop \in I}$ be a family of sets indexed by $I$.

Let $\family {I_\gamma}_{\gamma \mathop \in J}$ be a partitioning of $I$.

Then there exists a bijection:
 * $\ds \phi: \prod_{\gamma \mathop \in J} \paren {\prod_{\alpha \in \mathop I_\gamma} S_\alpha} \to \prod_{\alpha \mathop \in I} S_\alpha$

Proof
First a lemma:

We can define a projection $\pr_\gamma$:
 * $\ds \pr_\gamma: \prod_{\gamma \mathop \in J} \paren {\prod_{\alpha \in \mathop I_\gamma} S_\alpha} \to \prod_{\alpha \mathop \in I_\gamma} S_\alpha$

so that for $\ds X \in \prod_{\gamma \mathop \in J} \paren {\prod_{\alpha \in \mathop I_\gamma} S_\alpha}, X = \family {X_\gamma}_{\gamma \mathop \in J}$:
 * $\map {\pr_\gamma} X = X_\gamma$

From the lemma we can build $\phi$ so that for $I_\gamma = \set {\alpha: \alpha \in I_\gamma}$:


 * $\ds \map \phi {\mathbf x} = \prod_{\gamma \mathop \in J} \paren {\prod_{\alpha \mathop \in I_\gamma} \map {\psi_\alpha} {x_\alpha} }$

as $\phi$ uniquely sets all components of $\map {\pr_\gamma} X$ for all $\gamma$.

The result follows.