Editing Axiom of union (section)

== Formal statement ==
In the [[formal language]] of the Zermelo–Fraenkel axioms, the axiom reads:
:<math>\forall X\, \exists Y\, \forall u\, (u \in Y \leftrightarrow \exists z\, (u \in z \land z \in X))</math><ref>{{Cite book |last=Jech |first=Thomas J. |url=https://books.google.com/books?id=wmOISAAACAAJ |title=Set Theory |date=1997 |publisher=Springer |isbn=978-3-540-63048-7 |edition=2nd |pages=6 |language=en}}</ref>
or in words:
:[[Given any]] [[Set (mathematics)|set]] ''X'', [[Existential quantification|there is]] a set ''Y'' such that, for any element ''u'', ''u'' is a member of ''Y'' [[if and only if]] there is a set ''z'' such that ''u'' is a member of ''z'' [[logical conjunction|and]] ''z'' is a member of ''X''.
or, more simply:
:For any set <math>X</math>, there is a set <math>\bigcup X\ </math> which consists of just the elements of the elements of that set <math>X</math>.