In <firstname.lastname@example.org>, on 03/29/2013 at 06:07 AM, quasi <email@example.com> said:
>There are certain concepts for which sets are inadequate and classes >come to the rescue. For example, we need the class concept if we >want to define an equivalence on the collection of all groups, since >that collection is not a set.
There's no such set in ZFC, but there are set theories in which it exists aqnd is a set.
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