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Theorem pm4.57 483
Description: Theorem *4.57 of [WhiteheadRussell] p. 120. (Contributed by NM, 3-Jan-2005.)
Assertion
Ref Expression
pm4.57 (¬ (¬ φ ¬ ψ) ↔ (φ ψ))

Proof of Theorem pm4.57
StepHypRef Expression
1 oran 482 . 2 ((φ ψ) ↔ ¬ (¬ φ ¬ ψ))
21bicomi 193 1 (¬ (¬ φ ¬ ψ) ↔ (φ ψ))
Colors of variables: wff setvar class
Syntax hints:  ¬ wn 3  wb 176   wo 357   wa 358
This theorem was proved from axioms:  ax-1 5  ax-2 6  ax-3 7  ax-mp 8
This theorem depends on definitions:  df-bi 177  df-or 359  df-an 360
This theorem is referenced by:  nanbi  1294
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