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Theorem pm5.21im 338
Description: Two propositions are equivalent if they are both false. Closed form of 2false 339. Equivalent to a bi2 189-like version of the xor-connective. (Contributed by Wolf Lammen, 13-May-2013.)
Ref Expression

Proof of Theorem pm5.21im
StepHypRef Expression
1 nbn2 334 . 2
21biimpd 198 1
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:  pm5.21ndd  343  pm5.21  831
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