105 lines
3.5 KiB
Text
105 lines
3.5 KiB
Text
/-
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Copyright (c) 2015 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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Authors: Leonardo de Moura
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Choice function for decidable predicates on natural numbers.
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This module provides the following two declarations:
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choose {p : nat → Prop} [d : decidable_pred p] : (∃ x, p x) → nat
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choose_spec {p : nat → Prop} [d : decidable_pred p] (ex : ∃ x, p x) : p (choose ex)
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-/
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import data.nat.basic data.nat.order
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open nat subtype decidable well_founded
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namespace nat
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section find_x
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parameter {p : nat → Prop}
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private definition lbp (x : nat) : Prop := ∀ y, y < x → ¬ p y
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private lemma lbp_zero : lbp 0 :=
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λ y h, absurd h (not_lt_zero y)
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private lemma lbp_succ {x : nat} : lbp x → ¬ p x → lbp (succ x) :=
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λ lx npx y yltsx,
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or.elim (eq_or_lt_of_le (le_of_succ_le_succ yltsx))
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(suppose y = x, by substvars; assumption)
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(suppose y < x, lx y this)
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private definition gtb (a b : nat) : Prop :=
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a > b ∧ lbp a
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local infix ` ≺ `:50 := gtb
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private lemma acc_of_px {x : nat} : p x → acc gtb x :=
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assume h,
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acc.intro x (λ (y : nat) (l : y ≺ x),
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obtain (h₁ : y > x) (h₂ : ∀ a, a < y → ¬ p a), from l,
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absurd h (h₂ x h₁))
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private lemma acc_of_acc_succ {x : nat} : acc gtb (succ x) → acc gtb x :=
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assume h,
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acc.intro x (λ (y : nat) (l : y ≺ x),
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by_cases
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(suppose y = succ x, by substvars; assumption)
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(suppose y ≠ succ x,
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have x < y, from and.elim_left l,
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have succ x < y, from lt_of_le_and_ne this (ne.symm `y ≠ succ x`),
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acc.inv h (and.intro this (and.elim_right l))))
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private lemma acc_of_px_of_gt {x y : nat} : p x → y > x → acc gtb y :=
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assume px ygtx,
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acc.intro y (λ (z : nat) (l : z ≺ y),
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obtain (zgty : z > y) (h : ∀ a, a < z → ¬ p a), from l,
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absurd px (h x (lt.trans ygtx zgty)))
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private lemma acc_of_acc_of_lt : ∀ {x y : nat}, acc gtb x → y < x → acc gtb y
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| 0 y a0 ylt0 := absurd ylt0 !not_lt_zero
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| (succ x) y asx yltsx :=
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assert acc gtb x, from acc_of_acc_succ asx,
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by_cases
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(suppose y = x, by substvars; assumption)
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(suppose y ≠ x, acc_of_acc_of_lt `acc gtb x` (lt_of_le_and_ne (le_of_lt_succ yltsx) this))
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parameter (ex : ∃ a, p a)
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parameter [dp : decidable_pred p]
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include dp
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private lemma acc_of_ex (x : nat) : acc gtb x :=
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obtain (w : nat) (pw : p w), from ex,
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lt.by_cases
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(suppose x < w, acc_of_acc_of_lt (acc_of_px pw) this)
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(suppose x = w, by subst x; exact (acc_of_px pw))
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(suppose x > w, acc_of_px_of_gt pw this)
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private lemma wf_gtb : well_founded gtb :=
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well_founded.intro acc_of_ex
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private definition find.F (x : nat) : (Π x₁, x₁ ≺ x → lbp x₁ → {a : nat | p a}) → lbp x → {a : nat | p a} :=
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match x with
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| 0 := λ f l0, by_cases
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(λ p0 : p 0, tag 0 p0)
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(suppose ¬ p 0,
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have lbp 1, from lbp_succ l0 this,
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have 1 ≺ 0, from and.intro (lt.base 0) `lbp 1`,
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f 1 `1 ≺ 0` `lbp 1`)
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| (succ n) := λ f lsn, by_cases
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(suppose p (succ n), tag (succ n) this)
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(suppose ¬ p (succ n),
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have lss : lbp (succ (succ n)), from lbp_succ lsn this,
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have succ (succ n) ≺ succ n, from and.intro (lt.base (succ n)) lss,
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f (succ (succ n)) this lss)
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end
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private definition find_x : {x : nat | p x} :=
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@fix _ _ _ wf_gtb find.F 0 lbp_zero
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end find_x
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protected definition find {p : nat → Prop} [d : decidable_pred p] : (∃ x, p x) → nat :=
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assume h, elt_of (find_x h)
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protected theorem find_spec {p : nat → Prop} [d : decidable_pred p] (ex : ∃ x, p x) : p (nat.find ex) :=
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has_property (find_x ex)
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end nat
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