theorem nfsapp {x: nat} (F: set x) (a: nat x):
  $ FS/ x F $ >
  $ FN/ x a $ >
  $ FS/ x F @@ a $;
    | Step | Hyp | Ref | Expression | 
|---|---|---|---|
| 1 | hyp h1 | FS/ x F  | 
        |
| 2 | hyp h2 | FN/ x a  | 
        |
| 3 | nfnv | FN/ x a1  | 
        |
| 4 | 2, 3 | nfpr | FN/ x a, a1  | 
        
| 5 | 1, 4 | nfrapp | FS/ x F @' (a, a1)  | 
        
| 6 | 5 | nfsab | FS/ x S\ a1, F @' (a, a1)  | 
        
| 7 | 6 | conv sapp | FS/ x F @@ a  |