Description: Obsolete version of sb5 as of 21-Sep-2024. (Contributed by NM, 18-Aug-1993) (Proof modification is discouraged.) (New usage is discouraged.)
| Ref | Expression | ||
|---|---|---|---|
| Assertion | sb5OLD | |- ( [ y / x ] ph <-> E. x ( x = y /\ ph ) ) |
| Step | Hyp | Ref | Expression |
|---|---|---|---|
| 1 | nfs1v | |- F/ x [ y / x ] ph |
|
| 2 | sbequ12 | |- ( x = y -> ( ph <-> [ y / x ] ph ) ) |
|
| 3 | 1 2 | equsexv | |- ( E. x ( x = y /\ ph ) <-> [ y / x ] ph ) |
| 4 | 3 | bicomi | |- ( [ y / x ] ph <-> E. x ( x = y /\ ph ) ) |