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Theorem pm2.18OLD 129
Description: Obsolete version of pm2.18 128 as of 17-Nov-2023. (Contributed by NM, 29-Dec-1992.) (Proof modification is discouraged.) (New usage is discouraged.)
Assertion
Ref Expression
pm2.18OLD ((¬ 𝜑𝜑) → 𝜑)

Proof of Theorem pm2.18OLD
StepHypRef Expression
1 pm2.21 123 . . . 4 𝜑 → (𝜑 → ¬ (¬ 𝜑𝜑)))
21a2i 14 . . 3 ((¬ 𝜑𝜑) → (¬ 𝜑 → ¬ (¬ 𝜑𝜑)))
32con4d 115 . 2 ((¬ 𝜑𝜑) → ((¬ 𝜑𝜑) → 𝜑))
43pm2.43i 52 1 ((¬ 𝜑𝜑) → 𝜑)
Colors of variables: wff setvar class
Syntax hints:  ¬ wn 3  wi 4
This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8
This theorem is referenced by:  pm2.18dOLD  130
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