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Theorem 4exmid 977
 Description: The disjunction of the four possible combinations of two wffs and their negations is always true. A four-way excluded middle (see exmid 430). (Contributed by David Abernethy, 28-Jan-2014.)
Assertion
Ref Expression
4exmid (((𝜑𝜓) ∨ (¬ 𝜑 ∧ ¬ 𝜓)) ∨ ((𝜑 ∧ ¬ 𝜓) ∨ (𝜓 ∧ ¬ 𝜑)))

Proof of Theorem 4exmid
StepHypRef Expression
1 exmid 430 . 2 ((𝜑𝜓) ∨ ¬ (𝜑𝜓))
2 dfbi3 933 . . 3 ((𝜑𝜓) ↔ ((𝜑𝜓) ∨ (¬ 𝜑 ∧ ¬ 𝜓)))
3 xor 931 . . 3 (¬ (𝜑𝜓) ↔ ((𝜑 ∧ ¬ 𝜓) ∨ (𝜓 ∧ ¬ 𝜑)))
42, 3orbi12i 542 . 2 (((𝜑𝜓) ∨ ¬ (𝜑𝜓)) ↔ (((𝜑𝜓) ∨ (¬ 𝜑 ∧ ¬ 𝜓)) ∨ ((𝜑 ∧ ¬ 𝜓) ∨ (𝜓 ∧ ¬ 𝜑))))
51, 4mpbi 219 1 (((𝜑𝜓) ∨ (¬ 𝜑 ∧ ¬ 𝜓)) ∨ ((𝜑 ∧ ¬ 𝜓) ∨ (𝜓 ∧ ¬ 𝜑)))
 Colors of variables: wff setvar class Syntax hints:  ¬ wn 3   ↔ wb 195   ∨ wo 382   ∧ wa 383 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 196  df-or 384  df-an 385 This theorem is referenced by:  clsk1indlem3  37361
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