Theorem sbcom4 2088

Description: Commutativity law for substitution. This theorem was incorrectly used as our previous version of pm11.07 2089 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]𝜑 ↔ [𝑦 / 𝑥][𝑤 / 𝑧]𝜑)

Distinct variable groups:   𝜑,𝑥   𝜑,𝑧

Allowed substitution hints:   𝜑(𝑦,𝑤)

Proof of Theorem sbcom4

Step	Hyp	Ref	Expression
1		sbv 2087	. 2 ⊢ ([𝑤 / 𝑥]𝜑 ↔ 𝜑)
2		sbv 2087	. . 3 ⊢ ([𝑦 / 𝑧]𝜑 ↔ 𝜑)
3	2	sbbii 2075	. 2 ⊢ ([𝑤 / 𝑥][𝑦 / 𝑧]𝜑 ↔ [𝑤 / 𝑥]𝜑)
4		sbv 2087	. . . 4 ⊢ ([𝑤 / 𝑧]𝜑 ↔ 𝜑)
5	4	sbbii 2075	. . 3 ⊢ ([𝑦 / 𝑥][𝑤 / 𝑧]𝜑 ↔ [𝑦 / 𝑥]𝜑)
6		sbv 2087	. . 3 ⊢ ([𝑦 / 𝑥]𝜑 ↔ 𝜑)
7	5, 6	bitri 275	. 2 ⊢ ([𝑦 / 𝑥][𝑤 / 𝑧]𝜑 ↔ 𝜑)
8	1, 3, 7	3bitr4i 303	1 ⊢ ([𝑤 / 𝑥][𝑦 / 𝑧]𝜑 ↔ [𝑦 / 𝑥][𝑤 / 𝑧]𝜑)

Syntax hints:   ↔ wb 206  [wsb 2063

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1794  ax-4 1808  ax-5 1909  ax-6 1966

This theorem depends on definitions:  df-bi 207  df-ex 1779  df-sb 2064

This theorem is referenced by: (None)