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Theorem pm4.81 353
Description: Theorem *4.81 of [WhiteheadRussell] p. 122. (Contributed by NM, 3-Jan-2005.)
Assertion
Ref Expression
pm4.81  |-  ( ( -.  ph  ->  ph )  <->  ph )

Proof of Theorem pm4.81
StepHypRef Expression
1 pm2.18 101 . 2  |-  ( ( -.  ph  ->  ph )  ->  ph )
2 pm2.24 100 . 2  |-  ( ph  ->  ( -.  ph  ->  ph ) )
31, 2impbii 178 1  |-  ( ( -.  ph  ->  ph )  <->  ph )
Colors of variables: wff set class
Syntax hints:   -. wn 3    -> wi 4    <-> wb 174
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 175
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