Theorem sb9 1972

Description: Commutation of quantification and substitution variables. (Contributed by NM, 5-Aug-1993.) (Proof rewritten by Jim Kingdon, 23-Mar-2018.)

Assertion

Ref	Expression
sb9	|- ( A. x [ x / y ] ph <-> A. y [ y / x ] ph )

Proof of Theorem sb9

Dummy variable w is distinct from all other variables.

Step	Hyp	Ref	Expression
1		sb9v 1971	. . 3 |- ( A. y [ y / w ] [ w / x ] ph <-> A. w [ w / y ] [ w / x ] ph )
2		sbcom 1968	. . . 4 |- ( [ w / y ] [ w / x ] ph <-> [ w / x ] [ w / y ] ph )
3	2	albii 1463	. . 3 |- ( A. w [ w / y ] [ w / x ] ph <-> A. w [ w / x ] [ w / y ] ph )
4		sb9v 1971	. . 3 |- ( A. w [ w / x ] [ w / y ] ph <-> A. x [ x / w ] [ w / y ] ph )
5	1, 3, 4	3bitri 205	. 2 |- ( A. y [ y / w ] [ w / x ] ph <-> A. x [ x / w ] [ w / y ] ph )
6		ax-17 1519	. . . 4 |- ( ph -> A. w ph )
7	6	sbco2h 1957	. . 3 |- ( [ y / w ] [ w / x ] ph <-> [ y / x ] ph )
8	7	albii 1463	. 2 |- ( A. y [ y / w ] [ w / x ] ph <-> A. y [ y / x ] ph )
9	6	sbco2h 1957	. . 3 |- ( [ x / w ] [ w / y ] ph <-> [ x / y ] ph )
10	9	albii 1463	. 2 |- ( A. x [ x / w ] [ w / y ] ph <-> A. x [ x / y ] ph )
11	5, 8, 10	3bitr3ri 210	1 |- ( A. x [ x / y ] ph <-> A. y [ y / x ] ph )

Syntax hints:   <-> wb 104   A.wal 1346   [wsb 1755

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 704  ax-5 1440  ax-7 1441  ax-gen 1442  ax-ie1 1486  ax-ie2 1487  ax-8 1497  ax-10 1498  ax-11 1499  ax-i12 1500  ax-bndl 1502  ax-4 1503  ax-17 1519  ax-i9 1523  ax-ial 1527  ax-i5r 1528

This theorem depends on definitions:  df-bi 116  df-nf 1454  df-sb 1756

This theorem is referenced by:  sb9i  1973