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Theorem pm4.67 417
Description: Theorem *4.67 of [WhiteheadRussell] p. 120. (Contributed by NM, 3-Jan-2005.)
Assertion
Ref Expression
pm4.67 (¬ (¬ φ → ¬ ψ) ↔ (¬ φ ψ))

Proof of Theorem pm4.67
StepHypRef Expression
1 pm4.63 410 1 (¬ (¬ φ → ¬ ψ) ↔ (¬ φ ψ))
Colors of variables: wff setvar class
Syntax hints:  ¬ wn 3  wi 4  wb 176   wa 358
This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8
This theorem depends on definitions:  df-bi 177  df-an 360
This theorem is referenced by: (None)
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