Theorem elsng 3704

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	|- ( A e. V -> ( A e. { B } <-> A = B ) )

Proof of Theorem elsng

Dummy variable x is distinct from all other variables.

Step	Hyp	Ref	Expression
1		eqeq1 2239	. 2 |- ( x = A -> ( x = B <-> A = B ) )
2		df-sn 3695	. 2 |- { B } = { x | x = B }
3	1, 2	elab2g 2964	1 |- ( A e. V -> ( A e. { B } <-> A = B ) )

Syntax hints:   -> wi 4   <-> wb 105   = wceq 1398   e. wcel 2203   {csn 3689

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-ia1 106  ax-ia2 107  ax-ia3 108  ax-io 717  ax-5 1496  ax-7 1497  ax-gen 1498  ax-ie1 1542  ax-ie2 1543  ax-8 1553  ax-10 1554  ax-11 1555  ax-i12 1556  ax-bndl 1558  ax-4 1559  ax-17 1575  ax-i9 1579  ax-ial 1583  ax-i5r 1584  ax-ext 2214

This theorem depends on definitions:  df-bi 117  df-tru 1401  df-nf 1510  df-sb 1812  df-clab 2219  df-cleq 2225  df-clel 2228  df-nfc 2373  df-v 2815  df-sn 3695

This theorem is referenced by:  elsn  3705  elsni  3707  snidg  3718  eltpg  3734  eldifsn  3820  elsucg  4525  funconstss  5796  fniniseg  5798  fniniseg2  5800  suppimacnvfn  6446  tpfidceq  7190  fidcenumlemrks  7223  ltxr  10108  elfzp12  10433  1exp  10930  imasaddfnlemg  13527  0subm  13697  0subg  13916  0nsg  13931  kerf1ghm  13991  lsssn0  14518  plycj  15626  2lgslem2  15965