2014-07-19 19:09:47 +00:00
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-- Copyright (c) 2014 Microsoft Corporation. All rights reserved.
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-- Released under Apache 2.0 license as described in the file LICENSE.
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-- Author: Leonardo de Moura
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import logic
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namespace decidable
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inductive decidable (p : Bool) : Type :=
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| inl : p → decidable p
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| inr : ¬p → decidable p
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theorem induction_on {p : Bool} {C : Bool} (H : decidable p) (H1 : p → C) (H2 : ¬p → C) : C
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:= decidable_rec H1 H2 H
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theorem em (p : Bool) (H : decidable p) : p ∨ ¬p
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:= induction_on H (λ Hp, or_intro_left _ Hp) (λ Hnp, or_intro_right _ Hnp)
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definition rec [inline] {p : Bool} {C : Type} (H : decidable p) (H1 : p → C) (H2 : ¬p → C) : C
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:= decidable_rec H1 H2 H
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theorem decidable_true [instance] : decidable true
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:= inl trivial
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theorem decidable_false [instance] : decidable false
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:= inr not_false_trivial
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theorem decidable_and [instance] {a b : Bool} (Ha : decidable a) (Hb : decidable b) : decidable (a ∧ b)
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:= rec Ha
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(assume Ha : a, rec Hb
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(assume Hb : b, inl (and_intro Ha Hb))
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(assume Hnb : ¬b, inr (and_not_right a Hnb)))
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(assume Hna : ¬a, inr (and_not_left b Hna))
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theorem decidable_or [instance] {a b : Bool} (Ha : decidable a) (Hb : decidable b) : decidable (a ∨ b)
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:= rec Ha
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(assume Ha : a, inl (or_intro_left b Ha))
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(assume Hna : ¬a, rec Hb
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(assume Hb : b, inl (or_intro_right a Hb))
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(assume Hnb : ¬b, inr (or_not_intro Hna Hnb)))
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theorem decidable_not [instance] {a : Bool} (Ha : decidable a) : decidable (¬a)
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:= rec Ha
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(assume Ha, inr (not_not_intro Ha))
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(assume Hna, inl Hna)
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2014-07-20 00:09:37 +00:00
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theorem decidable_iff [instance] {a b : Bool} (Ha : decidable a) (Hb : decidable b) : decidable (a ↔ b)
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:= rec Ha
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(assume Ha, rec Hb
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(assume Hb : b, inl (iff_intro (assume H, Hb) (assume H, Ha)))
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(assume Hnb : ¬b, inr (not_intro (assume H : a ↔ b, absurd (iff_mp_left H Ha) Hnb))))
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(assume Hna, rec Hb
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(assume Hb : b, inr (not_intro (assume H : a ↔ b, absurd (iff_mp_right H Hb) Hna)))
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(assume Hnb : ¬b, inl (iff_intro (assume Ha, absurd_elim b Ha Hna) (assume Hb, absurd_elim a Hb Hnb))))
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theorem decidable_implies [instance] {a b : Bool} (Ha : decidable a) (Hb : decidable b) : decidable (a → b)
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:= rec Ha
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(assume Ha : a, rec Hb
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(assume Hb : b, inl (assume H, Hb))
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(assume Hnb : ¬b, inr (not_intro (assume H : a → b,
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absurd (H Ha) Hnb))))
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(assume Hna : ¬a, inl (assume Ha, absurd_elim b Ha Hna))
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