lean2/library/standard/diaconescu.lean

-- Copyright (c) 2014 Microsoft Corporation. All rights reserved.
-- Released under Apache 2.0 license as described in the file LICENSE.
-- Authors: Leonardo de Moura, Jeremy Avigad
import logic hilbert funext
using eq_proofs

-- Diaconescu’s theorem
-- Show that Excluded middle follows from
--   Hilbert's choice operator, function extensionality and Prop extensionality
section
hypothesis propext {a b : Prop} : (a → b) → (b → a) → a = b

parameter p : Prop

definition u [private] := epsilon (λ x, x = true ∨ p)

definition v [private] := epsilon (λ x, x = false ∨ p)

lemma u_def [private] : u = true ∨ p
:= epsilon_ax (exists_intro true (or_intro_left p (refl true)))

lemma v_def [private] : v = false ∨ p
:= epsilon_ax (exists_intro false (or_intro_left p (refl false)))

lemma uv_implies_p [private] : ¬(u = v) ∨ p
:= or_elim u_def
    (assume Hut : u = true, or_elim v_def
      (assume Hvf : v = false,
        have Hne : ¬(u = v), from Hvf⁻¹ ▸ Hut⁻¹ ▸ true_ne_false,
        or_intro_left p Hne)
      (assume Hp : p, or_intro_right (¬u = v) Hp))
    (assume Hp : p, or_intro_right (¬u = v) Hp)

lemma p_implies_uv [private] : p → u = v
:= assume Hp : p,
   have Hpred : (λ x, x = true ∨ p) = (λ x, x = false ∨ p), from
     funext (take x : Prop,
       have Hl : (x = true ∨ p) → (x = false ∨ p), from
         assume A, or_intro_right (x = false) Hp,
       have Hr : (x = false ∨ p) → (x = true ∨ p), from
         assume A, or_intro_right (x = true) Hp,
       show (x = true ∨ p) = (x = false ∨ p), from
         propext Hl Hr),
   show u = v, from
     Hpred ▸ (refl (epsilon (λ x, x = true ∨ p)))

theorem em : p ∨ ¬p
:= have H : ¬(u = v) → ¬p, from contrapos p_implies_uv,
   or_elim uv_implies_p
     (assume Hne : ¬(u = v), or_intro_right p (H Hne))
     (assume Hp : p, or_intro_left (¬p) Hp)
end