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Theorem elsng 3421
 Description: There is exactly one element in a singleton. Exercise 2 of [TakeutiZaring] p. 15 (generalized). (Contributed by NM, 13-Sep-1995.) (Proof shortened by Andrew Salmon, 29-Jun-2011.)
Assertion
Ref Expression
elsng (𝐴𝑉 → (𝐴 ∈ {𝐵} ↔ 𝐴 = 𝐵))

Proof of Theorem elsng
Dummy variable 𝑥 is distinct from all other variables.
StepHypRef Expression
1 eqeq1 2088 . 2 (𝑥 = 𝐴 → (𝑥 = 𝐵𝐴 = 𝐵))
2 df-sn 3412 . 2 {𝐵} = {𝑥𝑥 = 𝐵}
31, 2elab2g 2741 1 (𝐴𝑉 → (𝐴 ∈ {𝐵} ↔ 𝐴 = 𝐵))
 Colors of variables: wff set class Syntax hints:   → wi 4   ↔ wb 103   = wceq 1285   ∈ wcel 1434  {csn 3406 This theorem was proved from axioms:  ax-1 5  ax-2 6  ax-mp 7  ax-ia1 104  ax-ia2 105  ax-ia3 106  ax-io 663  ax-5 1377  ax-7 1378  ax-gen 1379  ax-ie1 1423  ax-ie2 1424  ax-8 1436  ax-10 1437  ax-11 1438  ax-i12 1439  ax-bndl 1440  ax-4 1441  ax-17 1460  ax-i9 1464  ax-ial 1468  ax-i5r 1469  ax-ext 2064 This theorem depends on definitions:  df-bi 115  df-tru 1288  df-nf 1391  df-sb 1687  df-clab 2069  df-cleq 2075  df-clel 2078  df-nfc 2209  df-v 2604  df-sn 3412 This theorem is referenced by:  elsn  3422  elsni  3424  snidg  3431  eltpg  3446  eldifsn  3525  elsucg  4167  funconstss  5317  fniniseg  5319  fniniseg2  5321  ltxr  8927  elfzp12  9192  1exp  9602
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