[FOM] A little trouble with definition of "binary relation" in Wikipedia (fwd)

Robert M. Solovay solovay at Math.Berkeley.EDU
Wed Jan 31 21:33:57 EST 2007

This doesn't work. The cartesian product of two factors [at least one 
of which is the null set] is null.

         --Bob Solovay

On Mon, 29 Jan 2007, Thomas Forster wrote:

>Yes, there are such theories but you don't need them. (In particular you
>don't need to know about Ackermann set theory!)  When you want an ordered
>pair of two NBG-style proper classes, use their (Wiener-Kuratowski)
>cartesian product instead: there is nothing sacred about the
>Wiener-Kuratowski implementation of pairing and unpairing.
>If you are unhappy about having a different implementation of
>pairing-and-unpairing for sets and for proper classes, then use the same
>(cartesian-product-using-W-K-pairs) for both!

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