Hi,

I am reading Naive Set Theory by Halmos and I can't wrap my head around this part:

Suppose that is a family of sets with domain ;write , and let be a familiy of sets with domain .

It is then not difficult to prove that

.

This is the generalized version of the associative law for unions.

Exercise: formulate and prove a generalized version of the commutative law.

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I do not see how that is the generalized version of the associative law for unions. Could someone please put this in dumber terms?

Thank you.