Theorem bj-nfs1

Description: Shorter proof of nfs1 (three essential steps instead of four). (Contributed by BJ, 2-May-2019) (Proof modification is discouraged.)

	Ref	Expression
Hypothesis	bj-nfs1.nf	$$⊢Ⅎy\phi$$
Assertion	bj-nfs1	$$⊢Ⅎx\left[y/x\right]\phi$$

Proof

Step	Hyp	Ref	Expression
1		bj-nfs1.nf	$$⊢Ⅎy\phi$$
2		bj-nfs1t2	$$⊢\forall xℲy\phi \to Ⅎx\left[y/x\right]\phi$$
3	2 1	mpg	$$⊢Ⅎx\left[y/x\right]\phi$$