Theorem elsng 3724

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 2245	. 2 |- ( x = A -> ( x = B <-> A = B ) )
2		df-sn 3715	. 2 |- { B } = { x | x = B }
3	1, 2	elab2g 2973	1 |- ( A e. V -> ( A e. { B } <-> A = B ) )

This proof depends on syntax axioms:   -> wi 4   <-> wb 105   = wceq 1402   e. wcel 2209   {csn 3709

This proof depends on axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-ia1 106  ax-ia2 107  ax-ia3 108  ax-io 721  ax-5 1500  ax-7 1501  ax-gen 1502  ax-ie1 1546  ax-ie2 1547  ax-8 1557  ax-10 1558  ax-11 1559  ax-i12 1560  ax-bndl 1562  ax-4 1563  ax-17 1579  ax-i9 1583  ax-ial 1587  ax-i5r 1588  ax-ext 2220

This proof depends on definitions:  df-bi 117  df-tru 1405  df-nf 1514  df-sb 1816  df-clab 2225  df-cleq 2231  df-clel 2234  df-nfc 2381  df-v 2823  df-sn 3715

This theorem is used by:  elsn  3725  elsni  3727  snidg  3738  eltpg  3754  eldifsn  3841  elsucg  4549  funconstss  5827  fniniseg  5829  fniniseg2  5831  suppimacnvfn  6486  tpfidceq  7237  fidcenumlemrks  7270  ltxr  10187  elfzp12  10516  1exp  11018  imasaddfnlemg  13684  0subm  13840  0subg  14051  0nsg  14066  kerf1ghm  14126  lsssn0  14756  plycj  15911  2lgslem2  16309