Description: Theorem *4.56 of WhiteheadRussell p. 120. (Contributed by NM, 3-Jan-2005)
Ref | Expression | ||
---|---|---|---|
Assertion | pm4.56 | ⊢ ( ( ¬ 𝜑 ∧ ¬ 𝜓 ) ↔ ¬ ( 𝜑 ∨ 𝜓 ) ) |
Step | Hyp | Ref | Expression |
---|---|---|---|
1 | ioran | ⊢ ( ¬ ( 𝜑 ∨ 𝜓 ) ↔ ( ¬ 𝜑 ∧ ¬ 𝜓 ) ) | |
2 | 1 | bicomi | ⊢ ( ( ¬ 𝜑 ∧ ¬ 𝜓 ) ↔ ¬ ( 𝜑 ∨ 𝜓 ) ) |