Theorem sbcom4 2208

Description: Commutativity law for substitution. This theorem was incorrectly used as our previous version of pm11.07 2209 but may still be useful. (Contributed by Andrew Salmon, 17-Jun-2011.) (Proof shortened by Jim Kingdon, 22-Jan-2018.)

Assertion

Ref	Expression
sbcom4	|- ( [ w / x ] [ y / z ] ph <-> [ y / x ] [ w / z ] ph )

Distinct variable groups:   ph, x, y, z   x, w, z

Allowed substitution hint:   ph( w)

Proof of Theorem sbcom4

Step	Hyp	Ref	Expression
1		nfv 1722	. . 3 |- F/ x ph
2	1	sbf 2139	. 2 |- ( [ w / x ] ph <-> ph )
3		nfv 1722	. . . 4 |- F/ z ph
4	3	sbf 2139	. . 3 |- ( [ y / z ] ph <-> ph )
5	4	sbbii 1764	. 2 |- ( [ w / x ] [ y / z ] ph <-> [ w / x ] ph )
6	3	sbf 2139	. . . 4 |- ( [ w / z ] ph <-> ph )
7	6	sbbii 1764	. . 3 |- ( [ y / x ] [ w / z ] ph <-> [ y / x ] ph )
8	1	sbf 2139	. . 3 |- ( [ y / x ] ph <-> ph )
9	7, 8	bitri 249	. 2 |- ( [ y / x ] [ w / z ] ph <-> ph )
10	2, 5, 9	3bitr4i 277	1 |- ( [ w / x ] [ y / z ] ph <-> [ y / x ] [ w / z ] ph )

Syntax hints:   <-> wb 184   [wsb 1757

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1633  ax-4 1646  ax-5 1719  ax-6 1765  ax-7 1808  ax-10 1855  ax-12 1872  ax-13 2020

This theorem depends on definitions:  df-bi 185  df-an 369  df-ex 1628  df-nf 1632  df-sb 1758

This theorem is referenced by: (None)