diff --git a/set.mm b/set.mm index c0b7d7019..5f7011fbd 100644 --- a/set.mm +++ b/set.mm @@ -24945,8 +24945,8 @@ the definition of class equality ( ~ df-cleq ). Its forward implication is called "class extensionality". Remark: the proof uses ~ axext4 to prove also the hypothesis of ~ df-cleq that is a degenerate instance, but it could be proved also from minimal propositional calculus and { - ~ ax-gen , ~ ax-4 }. (Contributed by NM, 15-Sep-1993.) (Revised by BJ, - 24-Jun-2019.) $) + ~ ax-gen , ~ ax-4 , ~ equid }. (Contributed by NM, 15-Sep-1993.) + (Revised by BJ, 24-Jun-2019.) $) dfcleq $p |- ( A = B <-> A. x ( x e. A <-> x e. B ) ) $= ( vy vz vv vu vt axext4 df-cleq ) ADEFGHBCDEGIHHFIJ $. $( $j usage 'dfcleq' avoids 'ax-8' 'ax-10' 'ax-11' 'ax-12' 'ax-13' @@ -25613,14 +25613,13 @@ the definition of class equality ( ~ df-cleq ). Its forward implication ${ $d z x $. $d z y $. $d z A $. $( Weaker version of ~ eleq1 (but more general than ~ elequ1 ) not - depending on ~ ax-ext nor ~ df-cleq . Note that this proof does not - depend on ~ ax-8 (while ~ eleq2w depends on ~ ax-9 ), so this also gives - a proof of ~ ax-8 from Tarski's FOL and ~ dfclel (simply consider an - instance where ` A ` is replaced by a setvar and deduce the forward - implication by ~ biimpd ), which shows that ~ dfclel is too powerful to - be used a definition instead of ~ df-clel . taking an instance of the - forward implication where a setvar is substituted for ` A ` (Contributed - by BJ, 24-Jun-2019.) $) + depending on ~ ax-ext nor ~ df-cleq . + + Note that this provides a proof of ~ ax-8 from Tarski's FOL and ~ dfclel + (simply consider an instance where ` A ` is replaced by a setvar and + deduce the forward implication by ~ biimpd ), which shows that ~ dfclel + is too powerful to be used as a definition instead of ~ df-clel . + (Contributed by BJ, 24-Jun-2019.) $) eleq1w $p |- ( x = y -> ( x e. A <-> y e. A ) ) $= ( vz weq cv wcel wa wex equequ2 anbi1d exbidv dfclel 3bitr4g ) ABEZDAEZDF CGZHZDIDBEZQHZDIAFZCGBFZCGORTDOPSQABDJKLDUACMDUBCMN $.