Theorem nfsb

Description: If z is not free in ph , it is not free in [ y / x ] ph when y and z are distinct. Usage of this theorem is discouraged because it depends on ax-13 . For a version requiring more disjoint variables, but fewer axioms, see nfsbv . (Contributed by Mario Carneiro, 11-Aug-2016) (Proof shortened by Wolf Lammen, 25-Feb-2024) (New usage is discouraged.)

	Ref	Expression
Hypothesis	nfsb.1	|- F/ z ph
Assertion	nfsb	|- F/ z [ y / x ] ph

Proof

Step	Hyp	Ref	Expression
1		nfsb.1	|- F/ z ph
2		nftru	|- F/ x T.
3	1	a1i	|- ( T. -> F/ z ph )
4	2 3	nfsbd	|- ( T. -> F/ z [ y / x ] ph )
5	4	mptru	|- F/ z [ y / x ] ph