By Martin Escardo, 2 September 2011.

Modified in December 2011 assuming the axiom of extensionality (which
is not used directly in this module, but instead in
GenericConvergentSequence).

We prove that the generic convergent sequence ℕ∞ is searchable, which
amounts to Theorem-3·6 of the paper

   http://www.cs.bham.ac.uk/~mhe/papers/omniscient.pdf, 
   http://www.cs.bham.ac.uk/~mhe/.talks/dagstuhl2011/omniscient.pdf

and conclude as a corollary that it is searchable and satisfies the
principle of omniscience.

\begin{code}

{-# OPTIONS --without-K #-}

module ConvergentSequenceSearchable where

\end{code}


(Continuity axioms and the fan principle are not assumed.)

\begin{code}

open import CurryHoward
open import Equality
open import Naturals
open import Two 
open import Cantor
open import GenericConvergentSequence
open import SetsAndFunctions
open import Searchable

\end{code}

This is the main theorem proved in this module:

\begin{code}

ℕ∞-is-searchable : searchable ℕ∞
ℕ∞-is-searchable p = ∃-intro a Lemma
 where 
  α : ₂ℕ 
  α 0       = p(under 0)
  α(succ n) = min₂ (α n) (p(under(succ n)))

  a : ℕ∞
  a = (α , λ i  Lemma[minab≤a])

  Dagger₀ : ∀(n : )  a  under n  p(under n)  
  Dagger₀ 0 r =  cong  w  incl w 0) r
  Dagger₀ (succ n) r = trans w t
   where 
    s : α n  
    s = trans (cong  w  incl w n) r) (under-diagonal₁ n)

    t : α(succ n)  
    t = trans (cong  w  incl w(succ n)) r) (under-diagonal₀ n)

    w : p(under(succ n))  α(succ n)
    w = sym(cong b  min₂ b (p(under(succ n)))) s)

  Dagger₁ : a    ∀(n : )  p(under n)  
  Dagger₁ r 0 = cong  w  incl w 0) r
  Dagger₁ r (succ n) = trans w t
   where 
    s : α n  
    s = cong  w  incl w n) r

    t : α(succ n)  
    t = cong  w  incl w (succ n)) r

    w : p(under(succ n))  α(succ n)
    w = sym(cong b  min₂ b (p(under(succ n)))) s)

  Claim₀ : p a    ∀(n : )  a  under n
  Claim₀ r n s = Lemma[b≡₁→b≢₀] r (Lemma s)
   where 
    Lemma : a  under n  p a  
    Lemma t = trans (cong p t) (Dagger₀ n t)

  Claim₁ : p a    a  
  Claim₁ r = not-ℕ-is-∞ (Claim₀ r) 

  Claim₂ : p a    ∀(n : )  p(under n)  
  Claim₂ r = Dagger₁(Claim₁ r)

  Claim₃ : p a    p   
  Claim₃ r = Lemma[x≡y→x≡z→z≡y] r (cong p (Claim₁ r))

  Lemma : p a    ∀(v : ℕ∞)  p v  
  Lemma r = ℕ∞-density (Claim₂ r) (Claim₃ r)

\end{code}

Corollaries:

\begin{code}

open import Omniscience

ℕ∞-is-omniscient : omniscient ℕ∞
ℕ∞-is-omniscient = searchable-implies-omniscient ℕ∞-is-searchable 

open import DiscreteAndSeparated

ℕ∞→ℕ-has-decidable-equality : discrete(ℕ∞  )
ℕ∞→ℕ-has-decidable-equality = omniscient-discrete-discrete ℕ∞-is-omniscient  u  ℕ-discrete)

\end{code}