Description: Obsolete version of sbcov as of 26-Aug-2025. (Contributed by NM, 14-May-1993) (Revised by GG, 7-Aug-2023) (Proof modification is discouraged.) (New usage is discouraged.)
| Ref | Expression | ||
|---|---|---|---|
| Assertion | sbcovOLD | |- ( [ y / x ] [ x / y ] ph <-> [ y / x ] ph ) |
| Step | Hyp | Ref | Expression |
|---|---|---|---|
| 1 | sbcom3vv | |- ( [ y / x ] [ x / y ] ph <-> [ y / x ] [ y / y ] ph ) |
|
| 2 | sbid | |- ( [ y / y ] ph <-> ph ) |
|
| 3 | 2 | sbbii | |- ( [ y / x ] [ y / y ] ph <-> [ y / x ] ph ) |
| 4 | 1 3 | bitri | |- ( [ y / x ] [ x / y ] ph <-> [ y / x ] ph ) |