Theorem snssOLD 3759

Description: Obsolete version of snss 3768 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 3650	. . . 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 1493	. 2 |- ( A. x ( x e. { A } -> x e. B ) <-> A. x ( x = A -> x e. B ) )
4		ssalel 3181	. 2 |- ( { A } C_ B <-> A. x ( x e. { A } -> x e. B ) )
5		snssOLD.1	. . 3 |- A e. _V
6	5	clel2 2906	. 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 1371   = wceq 1373   e. wcel 2176   _Vcvv 2772   C_ wss 3166   {csn 3633

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 711  ax-5 1470  ax-7 1471  ax-gen 1472  ax-ie1 1516  ax-ie2 1517  ax-8 1527  ax-10 1528  ax-11 1529  ax-i12 1530  ax-bndl 1532  ax-4 1533  ax-17 1549  ax-i9 1553  ax-ial 1557  ax-i5r 1558  ax-ext 2187

This theorem depends on definitions:  df-bi 117  df-tru 1376  df-nf 1484  df-sb 1786  df-clab 2192  df-cleq 2198  df-clel 2201  df-nfc 2337  df-v 2774  df-in 3172  df-ss 3179  df-sn 3639

This theorem is referenced by: (None)