Solution
= Solution
A choice <function> on a family $\mathcal A$ of nonempty sets satisfies $c(A)\in A$. Zermelo's theorem says every set has a total order under which every nonempty subset has a least element.
= Solution
A choice <function> on a family $\mathcal A$ of nonempty sets satisfies $c(A)\in A$. Zermelo's theorem says every set has a total order under which every nonempty subset has a least element.