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Theorem ssext 4198
Description: An extensionality-like principle that uses the subset instead of the membership relation: two classes are equal iff they have the same subsets. (Contributed by NM, 30-Jun-2004.)
Assertion
Ref Expression
ssext (𝐴 = 𝐵 ↔ ∀𝑥(𝑥𝐴𝑥𝐵))
Distinct variable groups:   𝑥,𝐴   𝑥,𝐵

Proof of Theorem ssext
StepHypRef Expression
1 ssextss 4197 . . 3 (𝐴𝐵 ↔ ∀𝑥(𝑥𝐴𝑥𝐵))
2 ssextss 4197 . . 3 (𝐵𝐴 ↔ ∀𝑥(𝑥𝐵𝑥𝐴))
31, 2anbi12i 456 . 2 ((𝐴𝐵𝐵𝐴) ↔ (∀𝑥(𝑥𝐴𝑥𝐵) ∧ ∀𝑥(𝑥𝐵𝑥𝐴)))
4 eqss 3156 . 2 (𝐴 = 𝐵 ↔ (𝐴𝐵𝐵𝐴))
5 albiim 1475 . 2 (∀𝑥(𝑥𝐴𝑥𝐵) ↔ (∀𝑥(𝑥𝐴𝑥𝐵) ∧ ∀𝑥(𝑥𝐵𝑥𝐴)))
63, 4, 53bitr4i 211 1 (𝐴 = 𝐵 ↔ ∀𝑥(𝑥𝐴𝑥𝐵))
Colors of variables: wff set class
Syntax hints:  wi 4  wa 103  wb 104  wal 1341   = wceq 1343  wss 3115
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-io 699  ax-5 1435  ax-7 1436  ax-gen 1437  ax-ie1 1481  ax-ie2 1482  ax-8 1492  ax-10 1493  ax-11 1494  ax-i12 1495  ax-bndl 1497  ax-4 1498  ax-17 1514  ax-i9 1518  ax-ial 1522  ax-i5r 1523  ax-14 2139  ax-ext 2147  ax-sep 4099  ax-pow 4152
This theorem depends on definitions:  df-bi 116  df-tru 1346  df-nf 1449  df-sb 1751  df-clab 2152  df-cleq 2158  df-clel 2161  df-nfc 2296  df-v 2727  df-in 3121  df-ss 3128  df-pw 3560  df-sn 3581
This theorem is referenced by: (None)
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