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Theorem axext4 2099
 Description: A bidirectional version of Extensionality. Although this theorem "looks" like it is just a definition of equality, it requires the Axiom of Extensionality for its proof under our axiomatization. See the comments for ax-ext 2097. (Contributed by NM, 14-Nov-2008.)
Assertion
Ref Expression
axext4 (𝑥 = 𝑦 ↔ ∀𝑧(𝑧𝑥𝑧𝑦))
Distinct variable groups:   𝑥,𝑧   𝑦,𝑧

Proof of Theorem axext4
StepHypRef Expression
1 elequ2 1674 . . 3 (𝑥 = 𝑦 → (𝑧𝑥𝑧𝑦))
21alrimiv 1828 . 2 (𝑥 = 𝑦 → ∀𝑧(𝑧𝑥𝑧𝑦))
3 axext3 2098 . 2 (∀𝑧(𝑧𝑥𝑧𝑦) → 𝑥 = 𝑦)
42, 3impbii 125 1 (𝑥 = 𝑦 ↔ ∀𝑧(𝑧𝑥𝑧𝑦))
 Colors of variables: wff set class Syntax hints:   ↔ wb 104  ∀wal 1312 This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-ia1 105  ax-ia2 106  ax-ia3 107  ax-5 1406  ax-gen 1408  ax-ie1 1452  ax-ie2 1453  ax-8 1465  ax-4 1470  ax-14 1475  ax-17 1489  ax-i9 1493  ax-ial 1497  ax-ext 2097 This theorem depends on definitions:  df-bi 116  df-nf 1420 This theorem is referenced by: (None)
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