Theorem nfsb4

Description: A variable not free in a proposition remains so after substitution in that proposition with a distinct variable. Usage of this theorem is discouraged because it depends on ax-13 . Theorem nfsb replaces the distinctor with a disjoint variable condition. Visit also nfsbv for a weaker version of nfsb not requiring ax-13 . (Contributed by NM, 14-May-1993) (Revised by Mario Carneiro, 4-Oct-2016) (New usage is discouraged.)

	Ref	Expression
Hypothesis	nfsb4.1	$$⊢Ⅎz\phi$$
Assertion	nfsb4	$$⊢\neg \forall zz=y\to Ⅎz\left[y/x\right]\phi$$

Proof

Step	Hyp	Ref	Expression
1		nfsb4.1	$$⊢Ⅎz\phi$$
2		nfsb4t	$$⊢\forall xℲz\phi \to \left(\neg \forall zz=y\to Ⅎz\left[y/x\right]\phi \right)$$
3	2 1	mpg	$$⊢\neg \forall zz=y\to Ⅎz\left[y/x\right]\phi$$