From: utzoo!decvax!harpo!ihnp4!ihuxr!lew
Newsgroups: net.math
Title: Look Ma! I'm comprehending!
Article-I.D.: ihuxr.322
Posted: Mon Feb  7 21:27:46 1983
Received: Wed Feb  9 05:34:27 1983
Reply-To: lew@ihuxr.UUCP (Lew Mammel, Jr.)

Thanks to Yigal Arens and Jerry Leichter for their rapid and informative
responses. I was confusing the substitution of y into a formula containing
x as a free variable, with a formula containing y as a free variable.

Jerry is right about the Monk book, he defines a set as "a class small
enough to be a member of some other class." He defines a universal
class, not a universal set.

I think it is interesting that the ZF formulation still lets us define
y={x: x y