theorem abid {x: nat} (p: wff x): $ x e. {x | p} <-> p $;
    | Step | Hyp | Ref | Expression | 
|---|---|---|---|
| 1 | bitr | (x e. {x | p} <-> [x / x] p) -> ([x / x] p <-> p) -> (x e. {x | p} <-> p) | |
| 2 | elab2 | x e. {x | p} <-> [x / x] p | |
| 3 | 1, 2 | ax_mp | ([x / x] p <-> p) -> (x e. {x | p} <-> p) | 
| 4 | sbid | [x / x] p <-> p | |
| 5 | 3, 4 | ax_mp | x e. {x | p} <-> p |