Theorem snssOLD 3748

Description: Obsolete version of snss 3757 as of 1-Jan-2025. (Contributed by NM, 5-Aug-1993.) (Proof modification is discouraged.) (New usage is discouraged.)

Hypothesis

Ref	Expression
snssOLD.1	|- A e. _V

Assertion

Ref	Expression
snssOLD	|- ( A e. B <-> { A } C_ B )

Proof of Theorem snssOLD

Dummy variable x is distinct from all other variables.

Step	Hyp	Ref	Expression
1		velsn 3639	. . . 4 |- ( x e. { A } <-> x = A )
2	1	imbi1i 238	. . 3 |- ( ( x e. { A } -> x e. B ) <-> ( x = A -> x e. B ) )
3	2	albii 1484	. 2 |- ( A. x ( x e. { A } -> x e. B ) <-> A. x ( x = A -> x e. B ) )
4		dfss2 3172	. 2 |- ( { A } C_ B <-> A. x ( x e. { A } -> x e. B ) )
5		snssOLD.1	. . 3 |- A e. _V
6	5	clel2 2897	. 2 |- ( A e. B <-> A. x ( x = A -> x e. B ) )
7	3, 4, 6	3bitr4ri 213	1 |- ( A e. B <-> { A } C_ B )

Syntax hints:   -> wi 4   <-> wb 105   A.wal 1362   = wceq 1364   e. wcel 2167   _Vcvv 2763   C_ wss 3157   {csn 3622

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 710  ax-5 1461  ax-7 1462  ax-gen 1463  ax-ie1 1507  ax-ie2 1508  ax-8 1518  ax-10 1519  ax-11 1520  ax-i12 1521  ax-bndl 1523  ax-4 1524  ax-17 1540  ax-i9 1544  ax-ial 1548  ax-i5r 1549  ax-ext 2178

This theorem depends on definitions:  df-bi 117  df-tru 1367  df-nf 1475  df-sb 1777  df-clab 2183  df-cleq 2189  df-clel 2192  df-nfc 2328  df-v 2765  df-in 3163  df-ss 3170  df-sn 3628

This theorem is referenced by: (None)