Theorem nfich1 47323

Description: The first interchangeable setvar variable is not free. (Contributed by AV, 21-Aug-2023.)

Assertion

Ref	Expression
nfich1	⊢ Ⅎ𝑥[𝑥⇄𝑦]𝜑

Proof of Theorem nfich1

Dummy variable 𝑎 is distinct from all other variables.

Step	Hyp	Ref	Expression
1		df-ich 47322	. 2 ⊢ ([𝑥⇄𝑦]𝜑 ↔ ∀𝑥∀𝑦([𝑥 / 𝑎][𝑦 / 𝑥][𝑎 / 𝑦]𝜑 ↔ 𝜑))
2		nfa1 2152	. 2 ⊢ Ⅎ𝑥∀𝑥∀𝑦([𝑥 / 𝑎][𝑦 / 𝑥][𝑎 / 𝑦]𝜑 ↔ 𝜑)
3	1, 2	nfxfr 1851	1 ⊢ Ⅎ𝑥[𝑥⇄𝑦]𝜑

Syntax hints:   ↔ wb 206  ∀wal 1535  Ⅎwnf 1781  [wsb 2064  [wich 47321

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1793  ax-4 1807  ax-10 2141

This theorem depends on definitions:  df-bi 207  df-or 847  df-ex 1778  df-nf 1782  df-ich 47322

This theorem is referenced by:  ichnfim  47340  ich2exprop  47347  ichreuopeq  47349