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Theorem eleq1ab 2715
Description: Extension (in the sense of Remark 3 of the comment of df-clab 2714) of elequ1 2114 from formulas of the form "setvar setvar" to formulas of the form "setvar class abstraction". This extension does not require ax-8 2109 contrary to elequ1 2114, but recall from Remark 3 of the comment of df-clab 2714 that it can be considered an extension only because of cvjust 2730, which does require ax-8 2109.

This is an instance of eleq1w 2823 where the containing class is a class abstraction, and contrary to it, it can be proved without df-clel 2815. See also eleq1 2828 for general classes.

The straightforward yet important fact that this statement can be proved from FOL= plus df-clab 2714 (hence without ax-ext 2707, df-cleq 2728 or df-clel 2815) was stressed by Mario Carneiro. (Contributed by BJ, 17-Aug-2023.)

Assertion
Ref Expression
eleq1ab (𝑥 = 𝑦 → (𝑥 ∈ {𝑧𝜑} ↔ 𝑦 ∈ {𝑧𝜑}))

Proof of Theorem eleq1ab
StepHypRef Expression
1 sbequ 2082 . 2 (𝑥 = 𝑦 → ([𝑥 / 𝑧]𝜑 ↔ [𝑦 / 𝑧]𝜑))
2 df-clab 2714 . 2 (𝑥 ∈ {𝑧𝜑} ↔ [𝑥 / 𝑧]𝜑)
3 df-clab 2714 . 2 (𝑦 ∈ {𝑧𝜑} ↔ [𝑦 / 𝑧]𝜑)
41, 2, 33bitr4g 314 1 (𝑥 = 𝑦 → (𝑥 ∈ {𝑧𝜑} ↔ 𝑦 ∈ {𝑧𝜑}))
Colors of variables: wff setvar class
Syntax hints:  wi 4  wb 206  [wsb 2063  wcel 2107  {cab 2713
This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1794  ax-4 1808  ax-5 1909  ax-6 1966  ax-7 2006
This theorem depends on definitions:  df-bi 207  df-an 396  df-ex 1779  df-sb 2064  df-clab 2714
This theorem is referenced by:  cleljustab  2716  ralab2  3702  rexab2  3704
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