Description: All setvar variables are sets (see isset ). Theorem 6.8 of Quine
p. 43. A shorter proof is possible from eleq2i but it uses more axioms.
(Contributed by NM, 26-May-1993) Remove use of ax-12 . (Revised by SN, 28-Aug-2023)(Proof shortened by BJ, 4-Sep-2024)