@mprv sets can have other sets as their members. In fact in pure set theory even integers are constructed of sets (nested combinations of null sets).

So taking the union of elements of sets is actually perfectly valid.

@freemo I agree, I've seen that construction before.

Reading the statement more carefully it looks like it's constructing a map between a set of sets (X is the set of sets x_i) and the union of those same sets.

Which is perfectly reasonable.

I misread the last symbol as an "equal" rather than a "belongs to". My mistake 😅

@mprv To be fair I have seen all these symbols before but it made me a bit dizzy and confused the first time i read it too :) Easy mistake.