13016 | The Axiom of Union dates from 1899, and seems fairly obvious [Cantor, by Maddy] |
13033 | Union: ∀F ∃A ∀Y ∀x (x ∈ Y ∧ Y ∈ F → x ∈ A) [Kunen] |
10876 | Unions: There is a set of all the elements which belong to at least one set in a collection [Clegg] |