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Theorem List for Metamath Proof Explorer - 35701-35800   *Has distinct variable group(s)
TypeLabelDescription
Statement
 
Theorembj-stdpc5 35701 More direct proof of stdpc5 2201. (Contributed by BJ, 15-Sep-2018.) (Proof modification is discouraged.)
𝑥𝜑       (∀𝑥(𝜑𝜓) → (𝜑 → ∀𝑥𝜓))
 
Theorem2stdpc5 35702 A double stdpc5 2201 (one direction of PM*11.3). See also 2stdpc4 2073 and 19.21vv 43125. (Contributed by BJ, 15-Sep-2018.) (Proof modification is discouraged.)
𝑥𝜑    &   𝑦𝜑       (∀𝑥𝑦(𝜑𝜓) → (𝜑 → ∀𝑥𝑦𝜓))
 
Theorembj-19.21t0 35703 Proof of 19.21t 2199 from stdpc5t 35700. (Contributed by BJ, 15-Sep-2018.) (Proof modification is discouraged.)
(Ⅎ𝑥𝜑 → (∀𝑥(𝜑𝜓) ↔ (𝜑 → ∀𝑥𝜓)))
 
Theoremexlimii 35704 Inference associated with exlimi 2210. Inferring a theorem when it is implied by an antecedent which may be true. (Contributed by BJ, 15-Sep-2018.)
𝑥𝜓    &   (𝜑𝜓)    &   𝑥𝜑       𝜓
 
Theoremax11-pm 35705 Proof of ax-11 2154 similar to PM's proof of alcom 2156 (PM*11.2). For a proof closer to PM's proof, see ax11-pm2 35709. Axiom ax-11 2154 is used in the proof only through nfa2 2170. (Contributed by BJ, 15-Sep-2018.) (Proof modification is discouraged.)
(∀𝑥𝑦𝜑 → ∀𝑦𝑥𝜑)
 
Theoremax6er 35706 Commuted form of ax6e 2382. (Could be placed right after ax6e 2382). (Contributed by BJ, 15-Sep-2018.)
𝑥 𝑦 = 𝑥
 
Theoremexlimiieq1 35707 Inferring a theorem when it is implied by an equality which may be true. (Contributed by BJ, 30-Sep-2018.)
𝑥𝜑    &   (𝑥 = 𝑦𝜑)       𝜑
 
Theoremexlimiieq2 35708 Inferring a theorem when it is implied by an equality which may be true. (Contributed by BJ, 15-Sep-2018.) (Revised by BJ, 30-Sep-2018.)
𝑦𝜑    &   (𝑥 = 𝑦𝜑)       𝜑
 
Theoremax11-pm2 35709* Proof of ax-11 2154 from the standard axioms of predicate calculus, similar to PM's proof of alcom 2156 (PM*11.2). This proof requires that 𝑥 and 𝑦 be distinct. Axiom ax-11 2154 is used in the proof only through nfal 2316, nfsb 2522, sbal 2159, sb8 2516. See also ax11-pm 35705. (Contributed by BJ, 15-Sep-2018.) (Proof modification is discouraged.)
(∀𝑥𝑦𝜑 → ∀𝑦𝑥𝜑)
 
21.17.4.16  Alternate definition of substitution
 
Theorembj-sbsb 35710 Biconditional showing two possible (dual) definitions of substitution df-sb 2068 not using dummy variables. (Contributed by BJ, 19-Mar-2021.)
(((𝑥 = 𝑦𝜑) ∧ ∃𝑥(𝑥 = 𝑦𝜑)) ↔ (∀𝑥(𝑥 = 𝑦𝜑) ∨ (𝑥 = 𝑦𝜑)))
 
Theorembj-dfsb2 35711 Alternate (dual) definition of substitution df-sb 2068 not using dummy variables. (Contributed by BJ, 19-Mar-2021.)
([𝑦 / 𝑥]𝜑 ↔ (∀𝑥(𝑥 = 𝑦𝜑) ∨ (𝑥 = 𝑦𝜑)))
 
21.17.4.17  Lemmas for substitution
 
Theorembj-sbf3 35712 Substitution has no effect on a bound variable (existential quantifier case); see sbf2 2263. (Contributed by BJ, 2-May-2019.)
([𝑦 / 𝑥]∃𝑥𝜑 ↔ ∃𝑥𝜑)
 
Theorembj-sbf4 35713 Substitution has no effect on a bound variable (nonfreeness case); see sbf2 2263. (Contributed by BJ, 2-May-2019.)
([𝑦 / 𝑥]Ⅎ𝑥𝜑 ↔ Ⅎ𝑥𝜑)
 
Theorembj-sbnf 35714* Move nonfree predicate in and out of substitution; see sbal 2159 and sbex 2277. (Contributed by BJ, 2-May-2019.)
([𝑧 / 𝑦]Ⅎ𝑥𝜑 ↔ Ⅎ𝑥[𝑧 / 𝑦]𝜑)
 
21.17.4.18  Existential uniqueness
 
Theorembj-eu3f 35715* Version of eu3v 2564 where the disjoint variable condition is replaced with a nonfreeness hypothesis. This is a "backup" of a theorem that used to be in the main part with label "eu3" and was deprecated in favor of eu3v 2564. (Contributed by NM, 8-Jul-1994.) (Proof shortened by BJ, 31-May-2019.)
𝑦𝜑       (∃!𝑥𝜑 ↔ (∃𝑥𝜑 ∧ ∃𝑦𝑥(𝜑𝑥 = 𝑦)))
 
21.17.4.19  First-order logic: miscellaneous

Miscellaneous theorems of first-order logic.

 
Theorembj-sblem1 35716* Lemma for substitution. (Contributed by BJ, 23-Jul-2023.)
(∀𝑥(𝜑 → (𝜓𝜒)) → (∀𝑥(𝜑𝜓) → (∃𝑥𝜑𝜒)))
 
Theorembj-sblem2 35717* Lemma for substitution. (Contributed by BJ, 23-Jul-2023.)
(∀𝑥(𝜑 → (𝜒𝜓)) → ((∃𝑥𝜑𝜒) → ∀𝑥(𝜑𝜓)))
 
Theorembj-sblem 35718* Lemma for substitution. (Contributed by BJ, 23-Jul-2023.)
(∀𝑥(𝜑 → (𝜓𝜒)) → (∀𝑥(𝜑𝜓) ↔ (∃𝑥𝜑𝜒)))
 
Theorembj-sbievw1 35719* Lemma for substitution. (Contributed by BJ, 23-Jul-2023.)
([𝑦 / 𝑥](𝜑𝜓) → ([𝑦 / 𝑥]𝜑𝜓))
 
Theorembj-sbievw2 35720* Lemma for substitution. (Contributed by BJ, 23-Jul-2023.)
([𝑦 / 𝑥](𝜓𝜑) → (𝜓 → [𝑦 / 𝑥]𝜑))
 
Theorembj-sbievw 35721* Lemma for substitution. Closed form of equsalvw 2007 and sbievw 2095. (Contributed by BJ, 23-Jul-2023.)
([𝑦 / 𝑥](𝜑𝜓) → ([𝑦 / 𝑥]𝜑𝜓))
 
Theorembj-sbievv 35722 Version of sbie 2501 with a second nonfreeness hypothesis and shorter proof. (Contributed by BJ, 18-Jul-2023.) (Proof modification is discouraged.)
𝑥𝜓    &   𝑦𝜑    &   (𝑥 = 𝑦 → (𝜑𝜓))       ([𝑦 / 𝑥]𝜑𝜓)
 
Theorembj-moeub 35723 Uniqueness is equivalent to existence being equivalent to unique existence. (Contributed by BJ, 14-Oct-2022.)
(∃*𝑥𝜑 ↔ (∃𝑥𝜑 ↔ ∃!𝑥𝜑))
 
Theorembj-sbidmOLD 35724 Obsolete proof of sbidm 2509 temporarily kept here to check it gives no additional insight. (Contributed by NM, 8-Mar-1995.) (Proof modification is discouraged.) (New usage is discouraged.)
([𝑦 / 𝑥][𝑦 / 𝑥]𝜑 ↔ [𝑦 / 𝑥]𝜑)
 
Theorembj-dvelimdv 35725* Deduction form of dvelim 2450 with disjoint variable conditions. Uncurried (imported) form of bj-dvelimdv1 35726. Typically, 𝑧 is a fresh variable used for the implicit substitution hypothesis that results in 𝜒 (namely, 𝜓 can be thought as 𝜓(𝑥, 𝑦) and 𝜒 as 𝜓(𝑥, 𝑧)). So the theorem says that if x is effectively free in 𝜓(𝑥, 𝑧), then if x and y are not the same variable, then 𝑥 is also effectively free in 𝜓(𝑥, 𝑦), in a context 𝜑.

One can weaken the implicit substitution hypothesis by adding the antecedent 𝜑 but this typically does not make the theorem much more useful. Similarly, one could use nonfreeness hypotheses instead of disjoint variable conditions but since this result is typically used when 𝑧 is a dummy variable, this would not be of much benefit. One could also remove DV (𝑥, 𝑧) since in the proof nfv 1917 can be replaced with nfal 2316 followed by nfn 1860.

Remark: nfald 2321 uses ax-11 2154; it might be possible to inline and use ax11w 2126 instead, but there is still a use via 19.12 2320 anyway. (Contributed by BJ, 20-Oct-2021.) (Proof modification is discouraged.)

(𝜑 → Ⅎ𝑥𝜒)    &   (𝑧 = 𝑦 → (𝜒𝜓))       ((𝜑 ∧ ¬ ∀𝑥 𝑥 = 𝑦) → Ⅎ𝑥𝜓)
 
Theorembj-dvelimdv1 35726* Curried (exported) form of bj-dvelimdv 35725 (of course, one is directly provable from the other, but we keep this proof for illustration purposes). (Contributed by BJ, 20-Oct-2021.) (Proof modification is discouraged.)
(𝜑 → Ⅎ𝑥𝜒)    &   (𝑧 = 𝑦 → (𝜒𝜓))       (𝜑 → (¬ ∀𝑥 𝑥 = 𝑦 → Ⅎ𝑥𝜓))
 
Theorembj-dvelimv 35727* A version of dvelim 2450 using the "nonfree" idiom. (Contributed by BJ, 20-Oct-2021.) (Proof modification is discouraged.)
𝑥𝜓    &   (𝑧 = 𝑦 → (𝜓𝜑))       (¬ ∀𝑥 𝑥 = 𝑦 → Ⅎ𝑥𝜑)
 
Theorembj-nfeel2 35728* Nonfreeness in a membership statement. (Contributed by BJ, 20-Oct-2021.) (Proof modification is discouraged.)
(¬ ∀𝑥 𝑥 = 𝑦 → Ⅎ𝑥 𝑦𝑧)
 
Theorembj-axc14nf 35729 Proof of a version of axc14 2462 using the "nonfree" idiom. (Contributed by BJ, 20-Oct-2021.) (Proof modification is discouraged.)
(¬ ∀𝑧 𝑧 = 𝑥 → (¬ ∀𝑧 𝑧 = 𝑦 → Ⅎ𝑧 𝑥𝑦))
 
Theorembj-axc14 35730 Alternate proof of axc14 2462 (even when inlining the above results, this gives a shorter proof). (Contributed by BJ, 20-Oct-2021.) (Proof modification is discouraged.)
(¬ ∀𝑧 𝑧 = 𝑥 → (¬ ∀𝑧 𝑧 = 𝑦 → (𝑥𝑦 → ∀𝑧 𝑥𝑦)))
 
TheoremmobidvALT 35731* Alternate proof of mobidv 2543 directly from its analogues albidv 1923 and exbidv 1924, using deduction style. Note the proof structure, similar to mobi 2541. (Contributed by Mario Carneiro, 7-Oct-2016.) Reduce axiom dependencies and shorten proof. Remove dependency on ax-6 1971, ax-7 2011, ax-12 2171 by adapting proof of mobid 2544. (Revised by BJ, 26-Sep-2022.) (Proof modification is discouraged.) (New usage is discouraged.)
(𝜑 → (𝜓𝜒))       (𝜑 → (∃*𝑥𝜓 ↔ ∃*𝑥𝜒))
 
Theoremsbn1ALT 35732 Alternate proof of sbn1 2105, not using the false constant. (Contributed by BJ, 18-Sep-2024.) (Proof modification is discouraged.) (New usage is discouraged.)
([𝑡 / 𝑥] ¬ 𝜑 → ¬ [𝑡 / 𝑥]𝜑)
 
21.17.5  Set theory
 
21.17.5.1  Eliminability of class terms

In this section, we give a sketch of the proof of the Eliminability Theorem for class terms in an extensional set theory where quantification occurs only over set variables.

Eliminability of class variables using the $a-statements ax-ext 2703, df-clab 2710, df-cleq 2724, df-clel 2810 is an easy result, proved for instance in Appendix X of Azriel Levy, Basic Set Theory, Dover Publications, 2002. Note that viewed from the set.mm axiomatization, it is a metatheorem not formalizable in set.mm. It states: every formula in the language of FOL + + class terms, but without class variables, is provably equivalent (over {FOL, ax-ext 2703, df-clab 2710, df-cleq 2724, df-clel 2810 }) to a formula in the language of FOL + (that is, without class terms).

The proof goes by induction on the complexity of the formula (see op. cit. for details). The base case is that of atomic formulas. The atomic formulas containing class terms are of one of the six following forms: for equality, 𝑥 = {𝑦𝜑}, {𝑥𝜑} = 𝑦, {𝑥𝜑} = {𝑦𝜓}, and for membership, 𝑦 ∈ {𝑥𝜑}, {𝑥𝜑} ∈ 𝑦, {𝑥𝜑} ∈ {𝑦𝜓}. These cases are dealt with by eliminable-veqab 35740, eliminable-abeqv 35741, eliminable-abeqab 35742, eliminable-velab 35739, eliminable-abelv 35743, eliminable-abelab 35744 respectively, which are all proved from {FOL, ax-ext 2703, df-clab 2710, df-cleq 2724, df-clel 2810 }.

(Details on the proof of the above six theorems. To understand how they were systematically proved, look at the theorems "eliminablei" below, which are special instances of df-clab 2710, dfcleq 2725 (proved from {FOL, ax-ext 2703, df-cleq 2724 }), and dfclel 2811 (proved from {FOL, df-clel 2810 }). Indeed, denote by (i) the formula proved by "eliminablei". One sees that the RHS of (1) has no class terms, the RHS's of (2x) have only class terms of the form dealt with by (1), and the RHS's of (3x) have only class terms of the forms dealt with by (1) and (2a). Note that in order to prove eliminable2a 35734, eliminable2b 35735 and eliminable3a 35737, we need to substitute a class variable for a setvar variable. This is possible because setvars are class terms: this is the content of the syntactic theorem cv 1540, which is used in these proofs (this does not appear in the html pages but it is in the set.mm file and you can check it using the Metamath program).)

The induction step relies on the fact that any formula is a FOL-combination of atomic formulas, so if one found equivalents for all atomic formulas constituting the formula, then the same FOL-combination of these equivalents will be equivalent to the original formula.

Note that one has a slightly more precise result: if the original formula has only class terms appearing in atomic formulas of the form 𝑦 ∈ {𝑥𝜑}, then df-clab 2710 is sufficient (over FOL) to eliminate class terms, and if the original formula has only class terms appearing in atomic formulas of the form 𝑦 ∈ {𝑥𝜑} and equalities, then df-clab 2710, ax-ext 2703 and df-cleq 2724 are sufficient (over FOL) to eliminate class terms.

To prove that { df-clab 2710, df-cleq 2724, df-clel 2810 } provides a definitional extension of {FOL, ax-ext 2703 }, one needs to prove both the above Eliminability Theorem, which compares the expressive powers of the languages with and without class terms, and the Conservativity Theorem, which compares the deductive powers when one adds { df-clab 2710, df-cleq 2724, df-clel 2810 }. It states that a formula without class terms is provable in one axiom system if and only if it is provable in the other, and that this remains true when one adds further definitions to {FOL, ax-ext 2703 }. It is also proved in op. cit. The proof is more difficult, since one has to construct for each proof of a statement without class terms, an associated proof not using { df-clab 2710, df-cleq 2724, df-clel 2810 }. It involves a careful case study on the structure of the proof tree.

 
Theoremeliminable1 35733 A theorem used to prove the base case of the Eliminability Theorem (see section comment). (Contributed by BJ, 19-Oct-2019.) (Proof modification is discouraged.) (New usage is discouraged.)
(𝑦 ∈ {𝑥𝜑} ↔ [𝑦 / 𝑥]𝜑)
 
Theoremeliminable2a 35734* A theorem used to prove the base case of the Eliminability Theorem (see section comment). (Contributed by BJ, 19-Oct-2019.) (Proof modification is discouraged.) (New usage is discouraged.)
(𝑥 = {𝑦𝜑} ↔ ∀𝑧(𝑧𝑥𝑧 ∈ {𝑦𝜑}))
 
Theoremeliminable2b 35735* A theorem used to prove the base case of the Eliminability Theorem (see section comment). (Contributed by BJ, 19-Oct-2019.) (Proof modification is discouraged.) (New usage is discouraged.)
({𝑥𝜑} = 𝑦 ↔ ∀𝑧(𝑧 ∈ {𝑥𝜑} ↔ 𝑧𝑦))
 
Theoremeliminable2c 35736* A theorem used to prove the base case of the Eliminability Theorem (see section comment). (Contributed by BJ, 19-Oct-2019.) (Proof modification is discouraged.) (New usage is discouraged.)
({𝑥𝜑} = {𝑦𝜓} ↔ ∀𝑧(𝑧 ∈ {𝑥𝜑} ↔ 𝑧 ∈ {𝑦𝜓}))
 
Theoremeliminable3a 35737* A theorem used to prove the base case of the Eliminability Theorem (see section comment). (Contributed by BJ, 19-Oct-2019.) (Proof modification is discouraged.) (New usage is discouraged.)
({𝑥𝜑} ∈ 𝑦 ↔ ∃𝑧(𝑧 = {𝑥𝜑} ∧ 𝑧𝑦))
 
Theoremeliminable3b 35738* A theorem used to prove the base case of the Eliminability Theorem (see section comment). (Contributed by BJ, 19-Oct-2019.) (Proof modification is discouraged.) (New usage is discouraged.)
({𝑥𝜑} ∈ {𝑦𝜓} ↔ ∃𝑧(𝑧 = {𝑥𝜑} ∧ 𝑧 ∈ {𝑦𝜓}))
 
Theoremeliminable-velab 35739 A theorem used to prove the base case of the Eliminability Theorem (see section comment): variable belongs to abstraction. (Contributed by BJ, 30-Apr-2024.) (Proof modification is discouraged.) (New usage is discouraged.)
(𝑦 ∈ {𝑥𝜑} ↔ [𝑦 / 𝑥]𝜑)
 
Theoremeliminable-veqab 35740* A theorem used to prove the base case of the Eliminability Theorem (see section comment): variable equals abstraction. (Contributed by BJ, 30-Apr-2024.) (Proof modification is discouraged.) (New usage is discouraged.)
(𝑥 = {𝑦𝜑} ↔ ∀𝑧(𝑧𝑥 ↔ [𝑧 / 𝑦]𝜑))
 
Theoremeliminable-abeqv 35741* A theorem used to prove the base case of the Eliminability Theorem (see section comment): abstraction equals variable. (Contributed by BJ, 30-Apr-2024.) Beware not to use symmetry of class equality. (Proof modification is discouraged.) (New usage is discouraged.)
({𝑥𝜑} = 𝑦 ↔ ∀𝑧([𝑧 / 𝑥]𝜑𝑧𝑦))
 
Theoremeliminable-abeqab 35742* A theorem used to prove the base case of the Eliminability Theorem (see section comment): abstraction equals abstraction. (Contributed by BJ, 30-Apr-2024.) (Proof modification is discouraged.) (New usage is discouraged.)
({𝑥𝜑} = {𝑦𝜓} ↔ ∀𝑧([𝑧 / 𝑥]𝜑 ↔ [𝑧 / 𝑦]𝜓))
 
Theoremeliminable-abelv 35743* A theorem used to prove the base case of the Eliminability Theorem (see section comment): abstraction belongs to variable. (Contributed by BJ, 30-Apr-2024.) (Proof modification is discouraged.) (New usage is discouraged.)
({𝑥𝜑} ∈ 𝑦 ↔ ∃𝑧(∀𝑡(𝑡𝑧 ↔ [𝑡 / 𝑥]𝜑) ∧ 𝑧𝑦))
 
Theoremeliminable-abelab 35744* A theorem used to prove the base case of the Eliminability Theorem (see section comment): abstraction belongs to abstraction. (Contributed by BJ, 30-Apr-2024.) (Proof modification is discouraged.) (New usage is discouraged.)
({𝑥𝜑} ∈ {𝑦𝜓} ↔ ∃𝑧(∀𝑡(𝑡𝑧 ↔ [𝑡 / 𝑥]𝜑) ∧ [𝑧 / 𝑦]𝜓))
 
21.17.5.2  Classes without the axiom of extensionality

A few results about classes can be proved without using ax-ext 2703. One could move all theorems from cab 2709 to df-clel 2810 (except for dfcleq 2725 and cvjust 2726) in a subsection "Classes" before the subsection on the axiom of extensionality, together with the theorems below. In that subsection, the last statement should be df-cleq 2724.

Note that without ax-ext 2703, the $a-statements df-clab 2710, df-cleq 2724, and df-clel 2810 are no longer eliminable (see previous section) (but PROBABLY df-clab 2710 is still conservative , while df-cleq 2724 and df-clel 2810 are not). This is not a reason not to study what is provable with them but without ax-ext 2703, in order to gauge their strengths more precisely.

Before that subsection, a subsection "The membership predicate" could group the statements with that are currently in the FOL part (including wcel 2106, wel 2107, ax-8 2108, ax-9 2116).

Remark: the weakening of eleq1 2821 / eleq2 2822 to eleq1w 2816 / eleq2w 2817 can also be done with eleq1i 2824, eqeltri 2829, eqeltrri 2830, eleq1a 2828, eleq1d 2818, eqeltrd 2833, eqeltrrd 2834, eqneltrd 2853, eqneltrrd 2854, nelneq 2857.

Remark: possibility to remove dependency on ax-10 2137, ax-11 2154, ax-13 2371 from nfcri 2890 and theorems using it if one adds a disjoint variable condition (that theorem is typically used with dummy variables, so the disjoint variable condition addition is not very restrictive), and then shorten nfnfc 2915.

 
Theorembj-denoteslem 35745* Lemma for bj-denotes 35746. (Contributed by BJ, 24-Apr-2024.) (Proof modification is discouraged.)
(∃𝑥 𝑥 = 𝐴𝐴 ∈ {𝑦 ∣ ⊤})
 
Theorembj-denotes 35746* This would be the justification theorem for the definition of the unary predicate "E!" by ( E! 𝐴 ↔ ∃𝑥𝑥 = 𝐴) which could be interpreted as "𝐴 exists" (as a set) or "𝐴 denotes" (in the sense of free logic).

A shorter proof using bitri 274 (to add an intermediate proposition 𝑧𝑧 = 𝐴 with a fresh 𝑧), cbvexvw 2040, and eqeq1 2736, requires the core axioms and { ax-9 2116, ax-ext 2703, df-cleq 2724 } whereas this proof requires the core axioms and { ax-8 2108, df-clab 2710, df-clel 2810 }.

Theorem bj-issetwt 35749 proves that "existing" is equivalent to being a member of a class abstraction. It also requires, with the present proof, { ax-8 2108, df-clab 2710, df-clel 2810 } (whereas with the shorter proof from cbvexvw 2040 and eqeq1 2736 it would require { ax-8 2108, ax-9 2116, ax-ext 2703, df-clab 2710, df-cleq 2724, df-clel 2810 }). That every class is equal to a class abstraction is proved by abid1 2870, which requires { ax-8 2108, ax-9 2116, ax-ext 2703, df-clab 2710, df-cleq 2724, df-clel 2810 }.

Note that there is no disjoint variable condition on 𝑥, 𝑦 but the theorem does not depend on ax-13 2371. Actually, the proof depends only on the logical axioms ax-1 6 through ax-7 2011 and sp 2176.

The symbol "E!" was chosen to be reminiscent of the analogous predicate in (inclusive or non-inclusive) free logic, which deals with the possibility of nonexistent objects. This analogy should not be taken too far, since here there are no equality axioms for classes: these are derived from ax-ext 2703 and df-cleq 2724 (e.g., eqid 2732 and eqeq1 2736). In particular, one cannot even prove 𝑥𝑥 = 𝐴𝐴 = 𝐴 without ax-ext 2703 and df-cleq 2724.

(Contributed by BJ, 29-Apr-2019.) (Proof modification is discouraged.)

(∃𝑥 𝑥 = 𝐴 ↔ ∃𝑦 𝑦 = 𝐴)
 
Theorembj-issettru 35747* Weak version of isset 3487 without ax-ext 2703. (Contributed by BJ, 24-Apr-2024.) (Proof modification is discouraged.)
(∃𝑥 𝑥 = 𝐴𝐴 ∈ {𝑦 ∣ ⊤})
 
Theorembj-elabtru 35748 This is as close as we can get to proving extensionality for "the" "universal" class without ax-ext 2703. (Contributed by BJ, 24-Apr-2024.) (Proof modification is discouraged.)
(𝐴 ∈ {𝑥 ∣ ⊤} ↔ 𝐴 ∈ {𝑦 ∣ ⊤})
 
Theorembj-issetwt 35749* Closed form of bj-issetw 35750. (Contributed by BJ, 29-Apr-2019.) (Proof modification is discouraged.)
(∀𝑥𝜑 → (𝐴 ∈ {𝑥𝜑} ↔ ∃𝑦 𝑦 = 𝐴))
 
Theorembj-issetw 35750* The closest one can get to isset 3487 without using ax-ext 2703. See also vexw 2715. Note that the only disjoint variable condition is between 𝑦 and 𝐴. From there, one can prove isset 3487 using eleq2i 2825 (which requires ax-ext 2703 and df-cleq 2724). (Contributed by BJ, 29-Apr-2019.) (Proof modification is discouraged.)
𝜑       (𝐴 ∈ {𝑥𝜑} ↔ ∃𝑦 𝑦 = 𝐴)
 
Theorembj-elissetALT 35751* Alternate proof of elisset 2815. This is essentially the same proof as seen by inlining bj-denotes 35746 and bj-denoteslem 35745. Use elissetv 2814 instead when sufficient (in particular when 𝑉 is substituted for V). (Contributed by BJ, 29-Apr-2019.) (Proof modification is discouraged.) (New usage is discouraged.)
(𝐴𝑉 → ∃𝑥 𝑥 = 𝐴)
 
Theorembj-issetiv 35752* Version of bj-isseti 35753 with a disjoint variable condition on 𝑥, 𝑉. The hypothesis uses 𝑉 instead of V for extra generality. This is indeed more general than isseti 3489 as long as elex 3492 is not available (and the non-dependence of bj-issetiv 35752 on special properties of the universal class V is obvious). Prefer its use over bj-isseti 35753 when sufficient (in particular when 𝑉 is substituted for V). (Contributed by BJ, 14-Sep-2019.) (Proof modification is discouraged.)
𝐴𝑉       𝑥 𝑥 = 𝐴
 
Theorembj-isseti 35753* Version of isseti 3489 with a class variable 𝑉 in the hypothesis instead of V for extra generality. This is indeed more general than isseti 3489 as long as elex 3492 is not available (and the non-dependence of bj-isseti 35753 on special properties of the universal class V is obvious). Use bj-issetiv 35752 instead when sufficient (in particular when 𝑉 is substituted for V). (Contributed by BJ, 13-Jun-2019.) (Proof modification is discouraged.)
𝐴𝑉       𝑥 𝑥 = 𝐴
 
Theorembj-ralvw 35754 A weak version of ralv 3498 not using ax-ext 2703 (nor df-cleq 2724, df-clel 2810, df-v 3476), and only core FOL axioms. See also bj-rexvw 35755. The analogues for reuv 3500 and rmov 3501 are not proved. (Contributed by BJ, 16-Jun-2019.) (Proof modification is discouraged.)
𝜓       (∀𝑥 ∈ {𝑦𝜓}𝜑 ↔ ∀𝑥𝜑)
 
Theorembj-rexvw 35755 A weak version of rexv 3499 not using ax-ext 2703 (nor df-cleq 2724, df-clel 2810, df-v 3476), and only core FOL axioms. See also bj-ralvw 35754. (Contributed by BJ, 16-Jun-2019.) (Proof modification is discouraged.)
𝜓       (∃𝑥 ∈ {𝑦𝜓}𝜑 ↔ ∃𝑥𝜑)
 
Theorembj-rababw 35756 A weak version of rabab 3502 not using df-clel 2810 nor df-v 3476 (but requiring ax-ext 2703) nor ax-12 2171. (Contributed by BJ, 16-Jun-2019.) (Proof modification is discouraged.)
𝜓       {𝑥 ∈ {𝑦𝜓} ∣ 𝜑} = {𝑥𝜑}
 
Theorembj-rexcom4bv 35757* Version of rexcom4b 3503 and bj-rexcom4b 35758 with a disjoint variable condition on 𝑥, 𝑉, hence removing dependency on df-sb 2068 and df-clab 2710 (so that it depends on df-clel 2810 and df-rex 3071 only on top of first-order logic). Prefer its use over bj-rexcom4b 35758 when sufficient (in particular when 𝑉 is substituted for V). Note the 𝑉 in the hypothesis instead of V. (Contributed by BJ, 14-Sep-2019.) (Proof modification is discouraged.)
𝐵𝑉       (∃𝑥𝑦𝐴 (𝜑𝑥 = 𝐵) ↔ ∃𝑦𝐴 𝜑)
 
Theorembj-rexcom4b 35758* Remove from rexcom4b 3503 dependency on ax-ext 2703 and ax-13 2371 (and on df-or 846, df-cleq 2724, df-nfc 2885, df-v 3476). The hypothesis uses 𝑉 instead of V (see bj-isseti 35753 for the motivation). Use bj-rexcom4bv 35757 instead when sufficient (in particular when 𝑉 is substituted for V). (Contributed by BJ, 16-Jun-2019.) (Proof modification is discouraged.)
𝐵𝑉       (∃𝑥𝑦𝐴 (𝜑𝑥 = 𝐵) ↔ ∃𝑦𝐴 𝜑)
 
Theorembj-ceqsalt0 35759 The FOL content of ceqsalt 3505. Lemma for bj-ceqsalt 35761 and bj-ceqsaltv 35762. (Contributed by BJ, 26-Sep-2019.) (Proof modification is discouraged.)
((Ⅎ𝑥𝜓 ∧ ∀𝑥(𝜃 → (𝜑𝜓)) ∧ ∃𝑥𝜃) → (∀𝑥(𝜃𝜑) ↔ 𝜓))
 
Theorembj-ceqsalt1 35760 The FOL content of ceqsalt 3505. Lemma for bj-ceqsalt 35761 and bj-ceqsaltv 35762. TODO: consider removing if it does not add anything to bj-ceqsalt0 35759. (Contributed by BJ, 26-Sep-2019.) (Proof modification is discouraged.)
(𝜃 → ∃𝑥𝜒)       ((Ⅎ𝑥𝜓 ∧ ∀𝑥(𝜒 → (𝜑𝜓)) ∧ 𝜃) → (∀𝑥(𝜒𝜑) ↔ 𝜓))
 
Theorembj-ceqsalt 35761* Remove from ceqsalt 3505 dependency on ax-ext 2703 (and on df-cleq 2724 and df-v 3476). Note: this is not doable with ceqsralt 3506 (or ceqsralv 3513), which uses eleq1 2821, but the same dependence removal is possible for ceqsalg 3507, ceqsal 3509, ceqsalv 3511, cgsexg 3518, cgsex2g 3519, cgsex4g 3520, ceqsex 3523, ceqsexv 3525, ceqsex2 3529, ceqsex2v 3530, ceqsex3v 3531, ceqsex4v 3532, ceqsex6v 3533, ceqsex8v 3534, gencbvex 3535 (after changing 𝐴 = 𝑦 to 𝑦 = 𝐴), gencbvex2 3536, gencbval 3537, vtoclgft 3540 (it uses , whose justification nfcjust 2884 does not use ax-ext 2703) and several other vtocl* theorems (see for instance bj-vtoclg1f 35793). See also bj-ceqsaltv 35762. (Contributed by BJ, 16-Jun-2019.) (Proof modification is discouraged.)
((Ⅎ𝑥𝜓 ∧ ∀𝑥(𝑥 = 𝐴 → (𝜑𝜓)) ∧ 𝐴𝑉) → (∀𝑥(𝑥 = 𝐴𝜑) ↔ 𝜓))
 
Theorembj-ceqsaltv 35762* Version of bj-ceqsalt 35761 with a disjoint variable condition on 𝑥, 𝑉, removing dependency on df-sb 2068 and df-clab 2710. Prefer its use over bj-ceqsalt 35761 when sufficient (in particular when 𝑉 is substituted for V). (Contributed by BJ, 16-Jun-2019.) (Proof modification is discouraged.)
((Ⅎ𝑥𝜓 ∧ ∀𝑥(𝑥 = 𝐴 → (𝜑𝜓)) ∧ 𝐴𝑉) → (∀𝑥(𝑥 = 𝐴𝜑) ↔ 𝜓))
 
Theorembj-ceqsalg0 35763 The FOL content of ceqsalg 3507. (Contributed by BJ, 12-Oct-2019.) (Proof modification is discouraged.)
𝑥𝜓    &   (𝜒 → (𝜑𝜓))       (∃𝑥𝜒 → (∀𝑥(𝜒𝜑) ↔ 𝜓))
 
Theorembj-ceqsalg 35764* Remove from ceqsalg 3507 dependency on ax-ext 2703 (and on df-cleq 2724 and df-v 3476). See also bj-ceqsalgv 35766. (Contributed by BJ, 12-Oct-2019.) (Proof modification is discouraged.)
𝑥𝜓    &   (𝑥 = 𝐴 → (𝜑𝜓))       (𝐴𝑉 → (∀𝑥(𝑥 = 𝐴𝜑) ↔ 𝜓))
 
Theorembj-ceqsalgALT 35765* Alternate proof of bj-ceqsalg 35764. (Contributed by BJ, 12-Oct-2019.) (Proof modification is discouraged.) (New usage is discouraged.)
𝑥𝜓    &   (𝑥 = 𝐴 → (𝜑𝜓))       (𝐴𝑉 → (∀𝑥(𝑥 = 𝐴𝜑) ↔ 𝜓))
 
Theorembj-ceqsalgv 35766* Version of bj-ceqsalg 35764 with a disjoint variable condition on 𝑥, 𝑉, removing dependency on df-sb 2068 and df-clab 2710. Prefer its use over bj-ceqsalg 35764 when sufficient (in particular when 𝑉 is substituted for V). (Contributed by BJ, 12-Oct-2019.) (Proof modification is discouraged.)
𝑥𝜓    &   (𝑥 = 𝐴 → (𝜑𝜓))       (𝐴𝑉 → (∀𝑥(𝑥 = 𝐴𝜑) ↔ 𝜓))
 
Theorembj-ceqsalgvALT 35767* Alternate proof of bj-ceqsalgv 35766. (Contributed by BJ, 12-Oct-2019.) (Proof modification is discouraged.) (New usage is discouraged.)
𝑥𝜓    &   (𝑥 = 𝐴 → (𝜑𝜓))       (𝐴𝑉 → (∀𝑥(𝑥 = 𝐴𝜑) ↔ 𝜓))
 
Theorembj-ceqsal 35768* Remove from ceqsal 3509 dependency on ax-ext 2703 (and on df-cleq 2724, df-v 3476, df-clab 2710, df-sb 2068). (Contributed by BJ, 12-Oct-2019.) (Proof modification is discouraged.)
𝑥𝜓    &   𝐴 ∈ V    &   (𝑥 = 𝐴 → (𝜑𝜓))       (∀𝑥(𝑥 = 𝐴𝜑) ↔ 𝜓)
 
Theorembj-ceqsalv 35769* Remove from ceqsalv 3511 dependency on ax-ext 2703 (and on df-cleq 2724, df-v 3476, df-clab 2710, df-sb 2068). (Contributed by BJ, 12-Oct-2019.) (Proof modification is discouraged.)
𝐴 ∈ V    &   (𝑥 = 𝐴 → (𝜑𝜓))       (∀𝑥(𝑥 = 𝐴𝜑) ↔ 𝜓)
 
Theorembj-spcimdv 35770* Remove from spcimdv 3583 dependency on ax-9 2116, ax-10 2137, ax-11 2154, ax-13 2371, ax-ext 2703, df-cleq 2724 (and df-nfc 2885, df-v 3476, df-or 846, df-tru 1544, df-nf 1786). For an even more economical version, see bj-spcimdvv 35771. (Contributed by BJ, 30-Nov-2020.) (Proof modification is discouraged.)
(𝜑𝐴𝐵)    &   ((𝜑𝑥 = 𝐴) → (𝜓𝜒))       (𝜑 → (∀𝑥𝜓𝜒))
 
Theorembj-spcimdvv 35771* Remove from spcimdv 3583 dependency on ax-7 2011, ax-8 2108, ax-10 2137, ax-11 2154, ax-12 2171 ax-13 2371, ax-ext 2703, df-cleq 2724, df-clab 2710 (and df-nfc 2885, df-v 3476, df-or 846, df-tru 1544, df-nf 1786) at the price of adding a disjoint variable condition on 𝑥, 𝐵 (but in usages, 𝑥 is typically a dummy, hence fresh, variable). For the version without this disjoint variable condition, see bj-spcimdv 35770. (Contributed by BJ, 3-Nov-2021.) (Proof modification is discouraged.)
(𝜑𝐴𝐵)    &   ((𝜑𝑥 = 𝐴) → (𝜓𝜒))       (𝜑 → (∀𝑥𝜓𝜒))
 
21.17.5.3  Characterization among sets versus among classes
 
Theoremelelb 35772 Equivalence between two common ways to characterize elements of a class 𝐵: the LHS says that sets are elements of 𝐵 if and only if they satisfy 𝜑 while the RHS says that classes are elements of 𝐵 if and only if they are sets and satisfy 𝜑. Therefore, the LHS is a characterization among sets while the RHS is a characterization among classes. Note that the LHS is often formulated using a class variable instead of the universe V while this is not possible for the RHS (apart from using 𝐵 itself, which would not be very useful). (Contributed by BJ, 26-Feb-2023.)
((𝐴 ∈ V → (𝐴𝐵𝜑)) ↔ (𝐴𝐵 ↔ (𝐴 ∈ V ∧ 𝜑)))
 
Theorembj-pwvrelb 35773 Characterization of the elements of the powerclass of the cartesian square of the universal class: they are exactly the sets which are binary relations. (Contributed by BJ, 16-Dec-2023.)
(𝐴 ∈ 𝒫 (V × V) ↔ (𝐴 ∈ V ∧ Rel 𝐴))
 
21.17.5.4  The nonfreeness quantifier for classes

In this section, we prove the symmetry of the nonfreeness quantifier for classes.

 
Theorembj-nfcsym 35774 The nonfreeness quantifier for classes defines a symmetric binary relation on var metavariables (irreflexivity is proved by nfnid 5373 with additional axioms; see also nfcv 2903). This could be proved from aecom 2426 and nfcvb 5374 but the latter requires a domain with at least two objects (hence uses extra axioms). (Contributed by BJ, 30-Sep-2018.) Proof modification is discouraged to avoid use of eqcomd 2738 instead of equcomd 2022; removing dependency on ax-ext 2703 is possible: prove weak versions (i.e. replace classes with setvars) of drnfc1 2922, eleq2d 2819 (using elequ2 2121), nfcvf 2932, dvelimc 2931, dvelimdc 2930, nfcvf2 2933. (Proof modification is discouraged.)
(𝑥𝑦𝑦𝑥)
 
21.17.5.5  Lemmas for class substitution

Some useful theorems for dealing with substitutions: sbbi 2304, sbcbig 3831, sbcel1g 4413, sbcel2 4415, sbcel12 4408, sbceqg 4409, csbvarg 4431.

 
Theorembj-sbeqALT 35775* Substitution in an equality (use the more general version bj-sbeq 35776 instead, without disjoint variable condition). (Contributed by BJ, 6-Oct-2018.) (New usage is discouraged.) (Proof modification is discouraged.)
([𝑦 / 𝑥]𝐴 = 𝐵𝑦 / 𝑥𝐴 = 𝑦 / 𝑥𝐵)
 
Theorembj-sbeq 35776 Distribute proper substitution through an equality relation. (See sbceqg 4409). (Contributed by BJ, 6-Oct-2018.)
([𝑦 / 𝑥]𝐴 = 𝐵𝑦 / 𝑥𝐴 = 𝑦 / 𝑥𝐵)
 
Theorembj-sbceqgALT 35777 Distribute proper substitution through an equality relation. Alternate proof of sbceqg 4409. (Contributed by BJ, 6-Oct-2018.) Proof modification is discouraged to avoid using sbceqg 4409, but the Metamath program "MM-PA> MINIMIZE_WITH * / EXCEPT sbceqg" command is ok. (Proof modification is discouraged.) (New usage is discouraged.)
(𝐴𝑉 → ([𝐴 / 𝑥]𝐵 = 𝐶𝐴 / 𝑥𝐵 = 𝐴 / 𝑥𝐶))
 
Theorembj-csbsnlem 35778* Lemma for bj-csbsn 35779 (in this lemma, 𝑥 cannot occur in 𝐴). (Contributed by BJ, 6-Oct-2018.) (New usage is discouraged.)
𝐴 / 𝑥{𝑥} = {𝐴}
 
Theorembj-csbsn 35779 Substitution in a singleton. (Contributed by BJ, 6-Oct-2018.)
𝐴 / 𝑥{𝑥} = {𝐴}
 
Theorembj-sbel1 35780* Version of sbcel1g 4413 when substituting a set. (Note: one could have a corresponding version of sbcel12 4408 when substituting a set, but the point here is that the antecedent of sbcel1g 4413 is not needed when substituting a set.) (Contributed by BJ, 6-Oct-2018.)
([𝑦 / 𝑥]𝐴𝐵𝑦 / 𝑥𝐴𝐵)
 
Theorembj-abv 35781 The class of sets verifying a tautology is the universal class. (Contributed by BJ, 24-Jul-2019.) (Proof modification is discouraged.)
(∀𝑥𝜑 → {𝑥𝜑} = V)
 
Theorembj-abvALT 35782 Alternate version of bj-abv 35781; shorter but uses ax-8 2108. (Contributed by BJ, 24-Jul-2019.) (Proof modification is discouraged.) (New usage is discouraged.)
(∀𝑥𝜑 → {𝑥𝜑} = V)
 
Theorembj-ab0 35783 The class of sets verifying a falsity is the empty set (closed form of abf 4402). (Contributed by BJ, 24-Jul-2019.) (Proof modification is discouraged.)
(∀𝑥 ¬ 𝜑 → {𝑥𝜑} = ∅)
 
Theorembj-abf 35784 Shorter proof of abf 4402 (which should be kept as abfALT). (Contributed by BJ, 24-Jul-2019.) (Proof modification is discouraged.)
¬ 𝜑       {𝑥𝜑} = ∅
 
Theorembj-csbprc 35785 More direct proof of csbprc 4406 (fewer essential steps). (Contributed by BJ, 24-Jul-2019.) (Proof modification is discouraged.)
𝐴 ∈ V → 𝐴 / 𝑥𝐵 = ∅)
 
21.17.5.6  Removing some axiom requirements and disjoint variable conditions
 
Theorembj-exlimvmpi 35786* A Fol lemma (exlimiv 1933 followed by mpi 20). (Contributed by BJ, 2-Jul-2022.) (Proof modification is discouraged.)
(𝜒 → (𝜑𝜓))    &   𝜑       (∃𝑥𝜒𝜓)
 
Theorembj-exlimmpi 35787 Lemma for bj-vtoclg1f1 35792 (an instance of this lemma is a version of bj-vtoclg1f1 35792 where 𝑥 and 𝑦 are identified). (Contributed by BJ, 30-Apr-2019.) (Proof modification is discouraged.)
𝑥𝜓    &   (𝜒 → (𝜑𝜓))    &   𝜑       (∃𝑥𝜒𝜓)
 
Theorembj-exlimmpbi 35788 Lemma for theorems of the vtoclg 3556 family. (Contributed by BJ, 3-Oct-2019.) (Proof modification is discouraged.)
𝑥𝜓    &   (𝜒 → (𝜑𝜓))    &   𝜑       (∃𝑥𝜒𝜓)
 
Theorembj-exlimmpbir 35789 Lemma for theorems of the vtoclg 3556 family. (Contributed by BJ, 3-Oct-2019.) (Proof modification is discouraged.)
𝑥𝜑    &   (𝜒 → (𝜑𝜓))    &   𝜓       (∃𝑥𝜒𝜑)
 
Theorembj-vtoclf 35790* Remove dependency on ax-ext 2703, df-clab 2710 and df-cleq 2724 (and df-sb 2068 and df-v 3476) from vtoclf 3547. (Contributed by BJ, 6-Oct-2019.) (Proof modification is discouraged.)
𝑥𝜓    &   𝐴𝑉    &   (𝑥 = 𝐴 → (𝜑𝜓))    &   𝜑       𝜓
 
Theorembj-vtocl 35791* Remove dependency on ax-ext 2703, df-clab 2710 and df-cleq 2724 (and df-sb 2068 and df-v 3476) from vtocl 3549. (Contributed by BJ, 6-Oct-2019.) (Proof modification is discouraged.)
𝐴𝑉    &   (𝑥 = 𝐴 → (𝜑𝜓))    &   𝜑       𝜓
 
Theorembj-vtoclg1f1 35792* The FOL content of vtoclg1f 3555 (hence not using ax-ext 2703, df-cleq 2724, df-nfc 2885, df-v 3476). Note the weakened "major" hypothesis and the disjoint variable condition between 𝑥 and 𝐴 (needed since the nonfreeness quantifier for classes is not available without ax-ext 2703; as a byproduct, this dispenses with ax-11 2154 and ax-13 2371). (Contributed by BJ, 30-Apr-2019.) (Proof modification is discouraged.)
𝑥𝜓    &   (𝑥 = 𝐴 → (𝜑𝜓))    &   𝜑       (∃𝑦 𝑦 = 𝐴𝜓)
 
Theorembj-vtoclg1f 35793* Reprove vtoclg1f 3555 from bj-vtoclg1f1 35792. This removes dependency on ax-ext 2703, df-cleq 2724 and df-v 3476. Use bj-vtoclg1fv 35794 instead when sufficient (in particular when 𝑉 is substituted for V). (Contributed by BJ, 14-Sep-2019.) (Proof modification is discouraged.)
𝑥𝜓    &   (𝑥 = 𝐴 → (𝜑𝜓))    &   𝜑       (𝐴𝑉𝜓)
 
Theorembj-vtoclg1fv 35794* Version of bj-vtoclg1f 35793 with a disjoint variable condition on 𝑥, 𝑉. This removes dependency on df-sb 2068 and df-clab 2710. Prefer its use over bj-vtoclg1f 35793 when sufficient (in particular when 𝑉 is substituted for V). (Contributed by BJ, 14-Sep-2019.) (Proof modification is discouraged.)
𝑥𝜓    &   (𝑥 = 𝐴 → (𝜑𝜓))    &   𝜑       (𝐴𝑉𝜓)
 
Theorembj-vtoclg 35795* A version of vtoclg 3556 with an additional disjoint variable condition (which is removable if we allow use of df-clab 2710, see bj-vtoclg1f 35793), which requires fewer axioms (i.e., removes dependency on ax-6 1971, ax-7 2011, ax-9 2116, ax-12 2171, ax-ext 2703, df-clab 2710, df-cleq 2724, df-v 3476). (Contributed by BJ, 2-Jul-2022.) (Proof modification is discouraged.)
(𝑥 = 𝐴 → (𝜑𝜓))    &   𝜑       (𝐴𝑉𝜓)
 
Theorembj-rabeqbid 35796 Version of rabeqbidv 3449 with two disjoint variable conditions removed and the third replaced by a nonfreeness hypothesis. (Contributed by BJ, 27-Apr-2019.)
𝑥𝜑    &   (𝜑𝐴 = 𝐵)    &   (𝜑 → (𝜓𝜒))       (𝜑 → {𝑥𝐴𝜓} = {𝑥𝐵𝜒})
 
Theorembj-seex 35797* Version of seex 5638 with a disjoint variable condition replaced by a nonfreeness hypothesis (for the sake of illustration). (Contributed by BJ, 27-Apr-2019.)
𝑥𝐵       ((𝑅 Se 𝐴𝐵𝐴) → {𝑥𝐴𝑥𝑅𝐵} ∈ V)
 
Theorembj-nfcf 35798* Version of df-nfc 2885 with a disjoint variable condition replaced with a nonfreeness hypothesis. (Contributed by BJ, 2-May-2019.)
𝑦𝐴       (𝑥𝐴 ↔ ∀𝑦𝑥 𝑦𝐴)
 
Theorembj-zfauscl 35799* General version of zfauscl 5301.

Remark: the comment in zfauscl 5301 is misleading: the essential use of ax-ext 2703 is the one via eleq2 2822 and not the one via vtocl 3549, since the latter can be proved without ax-ext 2703 (see bj-vtoclg 35795).

(Contributed by BJ, 2-Jul-2022.) (Proof modification is discouraged.)

(𝐴𝑉 → ∃𝑦𝑥(𝑥𝑦 ↔ (𝑥𝐴𝜑)))
 
21.17.5.7  Class abstractions

A few additional theorems on class abstractions and restricted class abstractions.

 
Theorembj-elabd2ALT 35800* Alternate proof of elabd2 3660 bypassing elab6g 3659 (and using sbiedvw 2096 instead of the 𝑥(𝑥 = 𝑦𝜓) idiom). (Contributed by BJ, 16-Oct-2024.) (Proof modification is discouraged.) (New usage is discouraged.)
(𝜑𝐴𝑉)    &   (𝜑𝐵 = {𝑥𝜓})    &   ((𝜑𝑥 = 𝐴) → (𝜓𝜒))       (𝜑 → (𝐴𝐵𝜒))
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