Description: Deduction associated with notnot and notnoti . (Contributed by Jarvin Udandy, 2-Sep-2016) Avoid biconditional. (Revised by Wolf Lammen, 27-Mar-2021)
| Ref | Expression | ||
|---|---|---|---|
| Hypothesis | notnotd.1 | |- ( ph -> ps ) |
|
| Assertion | notnotd | |- ( ph -> -. -. ps ) |
| Step | Hyp | Ref | Expression |
|---|---|---|---|
| 1 | notnotd.1 | |- ( ph -> ps ) |
|
| 2 | notnot | |- ( ps -> -. -. ps ) |
|
| 3 | 1 2 | syl | |- ( ph -> -. -. ps ) |