Theorem bj-sbf4

Description: Substitution has no effect on a bound variable (nonfreeness case); see sbf2 . (Contributed by BJ, 2-May-2019)

	Ref	Expression
Assertion	bj-sbf4	$$⊢\left[y/x\right]Ⅎx\phi \leftrightarrow Ⅎx\phi$$

Proof

Step	Hyp	Ref	Expression
1		nfnf1	$$⊢ℲxℲx\phi$$
2	1	sbf	$$⊢\left[y/x\right]Ⅎx\phi \leftrightarrow Ⅎx\phi$$