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Theorem elsng 3510
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 2122 . 2 (𝑥 = 𝐴 → (𝑥 = 𝐵𝐴 = 𝐵))
2 df-sn 3501 . 2 {𝐵} = {𝑥𝑥 = 𝐵}
31, 2elab2g 2802 1 (𝐴𝑉 → (𝐴 ∈ {𝐵} ↔ 𝐴 = 𝐵))
Colors of variables: wff set class
Syntax hints:  wi 4  wb 104   = wceq 1314  wcel 1463  {csn 3495
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 681  ax-5 1406  ax-7 1407  ax-gen 1408  ax-ie1 1452  ax-ie2 1453  ax-8 1465  ax-10 1466  ax-11 1467  ax-i12 1468  ax-bndl 1469  ax-4 1470  ax-17 1489  ax-i9 1493  ax-ial 1497  ax-i5r 1498  ax-ext 2097
This theorem depends on definitions:  df-bi 116  df-tru 1317  df-nf 1420  df-sb 1719  df-clab 2102  df-cleq 2108  df-clel 2111  df-nfc 2245  df-v 2660  df-sn 3501
This theorem is referenced by:  elsn  3511  elsni  3513  snidg  3522  eltpg  3537  eldifsn  3618  elsucg  4294  funconstss  5504  fniniseg  5506  fniniseg2  5508  fidcenumlemrks  6807  ltxr  9502  elfzp12  9819  1exp  10262
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