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Theorem pm4.56 805
Description: Theorem *4.56 of [WhiteheadRussell] p. 120. (Contributed by NM, 3-Jan-2005.)
Assertion
Ref Expression
pm4.56 ((¬ φ ¬ ψ) ↔ ¬ (φ ψ))

Proof of Theorem pm4.56
StepHypRef Expression
1 ioran 668 . 2 (¬ (φ ψ) ↔ (¬ φ ¬ ψ))
21bicomi 123 1 ((¬ φ ¬ ψ) ↔ ¬ (φ ψ))
Colors of variables: wff set class
Syntax hints:  ¬ wn 3   wa 97  wb 98   wo 628
This theorem was proved from axioms:  ax-1 5  ax-2 6  ax-mp 7  ax-ia1 99  ax-ia2 100  ax-ia3 101  ax-in1 544  ax-in2 545  ax-io 629
This theorem depends on definitions:  df-bi 110
This theorem is referenced by:  oranim  806  neanior  2286  prneimg  3536  nqnq0pi  6420
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