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Metamath Proof Explorer
Description: Theorem *4.67 of WhiteheadRussell p. 120. (Contributed by NM, 3-Jan-2005)
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|
Ref |
Expression |
|
Assertion |
pm4.67 |
⊢ ( ¬ ( ¬ 𝜑 → ¬ 𝜓 ) ↔ ( ¬ 𝜑 ∧ 𝜓 ) ) |
Proof
| Step |
Hyp |
Ref |
Expression |
| 1 |
|
pm4.63 |
⊢ ( ¬ ( ¬ 𝜑 → ¬ 𝜓 ) ↔ ( ¬ 𝜑 ∧ 𝜓 ) ) |