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Theorem elprg 4304
 Description: A member of an unordered pair of classes is one or the other of them. Exercise 1 of [TakeutiZaring] p. 15, generalized. (Contributed by NM, 13-Sep-1995.)
Assertion
Ref Expression
elprg (𝐴𝑉 → (𝐴 ∈ {𝐵, 𝐶} ↔ (𝐴 = 𝐵𝐴 = 𝐶)))

Proof of Theorem elprg
Dummy variable 𝑥 is distinct from all other variables.
StepHypRef Expression
1 eqeq1 2728 . . 3 (𝑥 = 𝐴 → (𝑥 = 𝐵𝐴 = 𝐵))
2 eqeq1 2728 . . 3 (𝑥 = 𝐴 → (𝑥 = 𝐶𝐴 = 𝐶))
31, 2orbi12d 748 . 2 (𝑥 = 𝐴 → ((𝑥 = 𝐵𝑥 = 𝐶) ↔ (𝐴 = 𝐵𝐴 = 𝐶)))
4 dfpr2 4303 . 2 {𝐵, 𝐶} = {𝑥 ∣ (𝑥 = 𝐵𝑥 = 𝐶)}
53, 4elab2g 3458 1 (𝐴𝑉 → (𝐴 ∈ {𝐵, 𝐶} ↔ (𝐴 = 𝐵𝐴 = 𝐶)))
 Colors of variables: wff setvar class Syntax hints:   → wi 4   ↔ wb 196   ∨ wo 382   = wceq 1596   ∈ wcel 2103  {cpr 4287 This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1835  ax-4 1850  ax-5 1952  ax-6 2018  ax-7 2054  ax-9 2112  ax-10 2132  ax-11 2147  ax-12 2160  ax-13 2355  ax-ext 2704 This theorem depends on definitions:  df-bi 197  df-or 384  df-an 385  df-tru 1599  df-ex 1818  df-nf 1823  df-sb 2011  df-clab 2711  df-cleq 2717  df-clel 2720  df-nfc 2855  df-v 3306  df-un 3685  df-sn 4286  df-pr 4288 This theorem is referenced by:  elpri  4305  elpr  4306  elpr2  4307  elpr2OLD  4308  eldifpr  4312  eltpg  4334  ifpr  4340  prid1g  4402  ssprss  4464  preq1b  4485  ordunpr  7143  hashtpg  13380  cnsubrg  19929  atandm  24723  1egrvtxdg0  26538  eupth2lem1  27291  eliccioo  29869  nelpr2  39677  nelpr1  39678  sfprmdvdsmersenne  41947
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