theorem nfsb1 {x: nat} (a: nat) (b: wff x): $ F/ x [a / x] b $;
| Step | Hyp | Ref | Expression |
|---|---|---|---|
| 1 | nfv | F/ x y = a |
|
| 2 | nfal1 | F/ x A. x (x = y -> b) |
|
| 3 | 1, 2 | nfim | F/ x y = a -> A. x (x = y -> b) |
| 4 | 3 | nfal | F/ x A. y (y = a -> A. x (x = y -> b)) |
| 5 | 4 | conv sb | F/ x [a / x] b |