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Theorem oridm 500
Description: Idempotent law for disjunction. Theorem *4.25 of [WhiteheadRussell] p. 117. (Contributed by NM, 5-Aug-1993.) (Proof shortened by Andrew Salmon, 16-Apr-2011.) (Proof shortened by Wolf Lammen, 10-Mar-2013.)
Assertion
Ref Expression
oridm

Proof of Theorem oridm
StepHypRef Expression
1 pm1.2 499 . 2
2 pm2.07 385 . 2
31, 2impbii 180 1
Colors of variables: wff setvar class
Syntax hints:   wb 176   wo 357
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 177  df-or 359
This theorem is referenced by:  pm4.25  501  orordi  516  orordir  517  truortru  1340  falorfal  1343  unidm  3407
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