Theorem oridm 759

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	|- ( ( ph \/ ph ) <-> ph )

Proof of Theorem oridm

Step	Hyp	Ref	Expression
1		pm1.2 758	. 2 |- ( ( ph \/ ph ) -> ph )
2		pm2.07 739	. 2 |- ( ph -> ( ph \/ ph ) )
3	1, 2	impbii 126	1 |- ( ( ph \/ ph ) <-> ph )

Syntax hints:   <-> wb 105   \/ wo 710

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-ia1 106  ax-ia2 107  ax-ia3 108  ax-io 711

This theorem depends on definitions:  df-bi 117

This theorem is referenced by:  pm4.25  760  orordi  775  orordir  776  truortru  1425  falorfal  1428  truxortru  1439  falxorfal  1442  unidm  3316  preqsn  3816  funopsn  5762  reapirr  8650  reapti  8652  lt2msq  8959  nn0ge2m1nn  9355  absext  11374  prmdvdsexp  12470  sqpweven  12497  2sqpwodd  12498