theorem reseq (_A1 _A2 _B1 _B2: set): $ _A1 == _A2 -> _B1 == _B2 -> _A1 |` _B1 == _A2 |` _B2 $;
| Step | Hyp | Ref | Expression | 
|---|---|---|---|
| 1 | anl | _A1 == _A2 /\ _B1 == _B2 -> _A1 == _A2 | |
| 2 | anr | _A1 == _A2 /\ _B1 == _B2 -> _B1 == _B2 | |
| 3 | 1, 2 | reseqd | _A1 == _A2 /\ _B1 == _B2 -> _A1 |` _B1 == _A2 |` _B2 | 
| 4 | 3 | exp | _A1 == _A2 -> _B1 == _B2 -> _A1 |` _B1 == _A2 |` _B2 |