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Theorem con1b 323
Description: Contraposition. Bidirectional version of con1 120. (Contributed by NM, 5-Aug-1993.)
Assertion
Ref Expression
con1b ((¬ φψ) ↔ (¬ ψφ))

Proof of Theorem con1b
StepHypRef Expression
1 con1 120 . 2 ((¬ φψ) → (¬ ψφ))
2 con1 120 . 2 ((¬ ψφ) → (¬ φψ))
31, 2impbii 180 1 ((¬ φψ) ↔ (¬ ψφ))
Colors of variables: wff setvar class
Syntax hints:  ¬ wn 3  wi 4  wb 176
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
This theorem is referenced by: (None)
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