Theorem wl-dfcleq.basic 37951

Description: This theorem is a conservative extension of ax-ext 2728 to classes, with no hypotheses. It is not complete, since ax-8 2138 can be derived (see in-ax8 36532) via alpha-renaming.

Although unsuitable for general use, it is adequate for the development of theorems unaffected by alpha-renaming, including:

1. Theorems with no bound variables in the hypotheses or conclusion (see eqriv 2753).

2. Theorems using the same bound variable throughout (see abbib 2825).

3. Theorems with distinct bound variables arising only through implicit substitution (see eqabbw 2829).

Remark: the proof uses axextb 2731 to prove the hypothesis of df-cleq 2748 that is a degenerate instance, but it could be proved also from minimal propositional calculus and { ax-gen 1809, equid 2026 }. (Contributed by NM, 15-Sep-1993.) (Revised by BJ, 24-Jun-2019.)

Assertion

Ref	Expression
wl-dfcleq.basic	⊢ (𝐴 = 𝐵 ↔ ∀𝑥(𝑥 ∈ 𝐴 ↔ 𝑥 ∈ 𝐵))

Distinct variable groups:   𝑥,𝐴   𝑥,𝐵

Proof of Theorem wl-dfcleq.basic

Dummy variables 𝑦 𝑧 𝑡 𝑢 𝑣 are mutually distinct and distinct from all other variables.

Step	Hyp	Ref	Expression
1		axextb 2731	. 2 ⊢ (𝑦 = 𝑧 ↔ ∀𝑢(𝑢 ∈ 𝑦 ↔ 𝑢 ∈ 𝑧))
2		axextb 2731	. 2 ⊢ (𝑡 = 𝑡 ↔ ∀𝑣(𝑣 ∈ 𝑡 ↔ 𝑣 ∈ 𝑡))
3	1, 2	wl-df.cleq 37950	1 ⊢ (𝐴 = 𝐵 ↔ ∀𝑥(𝑥 ∈ 𝐴 ↔ 𝑥 ∈ 𝐵))

Syntax hints:   ↔ wb 208  ∀wal 1552   = wceq 1554   ∈ wcel 2136

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1809  ax-4 1823  ax-5 1924  ax-6 1981  ax-7 2022  ax-9 2146  ax-ext 2728

This theorem depends on definitions:  df-bi 209  df-an 399  df-ex 1794  df-cleq 2748

This theorem is referenced by:  wl-dfcleq.just  37952  wl-dfcleq  37956