lean2/hott/types/W.hlean

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/-
Copyright (c) 2014 Floris van Doorn. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Module: types.W
Author: Floris van Doorn
Theorems about W-types (well-founded trees)
-/
import .sigma .pi
open eq sigma sigma.ops equiv is_equiv
inductive Wtype.{l k} {A : Type.{l}} (B : A → Type.{k}) : Type.{max l k} :=
sup : Π (a : A), (B a → Wtype.{l k} B) → Wtype.{l k} B
namespace Wtype
notation `W` binders `,` r:(scoped B, Wtype B) := r
universe variables u v
variables {A A' : Type.{u}} {B B' : A → Type.{v}} {C : Π(a : A), B a → Type}
{a a' : A} {f : B a → W a, B a} {f' : B a' → W a, B a} {w w' : W(a : A), B a}
protected definition pr1 [unfold-c 3] (w : W(a : A), B a) : A :=
by cases w with a f; exact a
protected definition pr2 [unfold-c 3] (w : W(a : A), B a) : B (Wtype.pr1 w) → W(a : A), B a :=
by cases w with a f; exact f
namespace ops
postfix `.1`:(max+1) := Wtype.pr1
postfix `.2`:(max+1) := Wtype.pr2
notation `⟨` a `,` f `⟩`:0 := Wtype.sup a f --input ⟨ ⟩ as \< \>
end ops
open ops
protected definition eta (w : W a, B a) : ⟨w.1 , w.2⟩ = w :=
by cases w; exact idp
definition sup_eq_sup (p : a = a') (q : p ▸ f = f') : ⟨a, f⟩ = ⟨a', f'⟩ :=
by cases p; cases q; exact idp
definition Wtype_eq (p : w.1 = w'.1) (q : p ▸ w.2 = w'.2) : w = w' :=
by cases w; cases w';exact (sup_eq_sup p q)
definition Wtype_eq_pr1 (p : w = w') : w.1 = w'.1 :=
by cases p;exact idp
definition Wtype_eq_pr2 (p : w = w') : Wtype_eq_pr1 p ▸ w.2 = w'.2 :=
by cases p;exact idp
namespace ops
postfix `..1`:(max+1) := Wtype_eq_pr1
postfix `..2`:(max+1) := Wtype_eq_pr2
end ops open ops open sigma
definition sup_path_W (p : w.1 = w'.1) (q : p ▸ w.2 = w'.2)
: ⟨(Wtype_eq p q)..1,(Wtype_eq p q)..2⟩ = ⟨p, q⟩ :=
by cases w; cases w'; cases p; cases q; exact idp
definition pr1_path_W (p : w.1 = w'.1) (q : p ▸ w.2 = w'.2) : (Wtype_eq p q)..1 = p :=
!sup_path_W..1
definition pr2_path_W (p : w.1 = w'.1) (q : p ▸ w.2 = w'.2)
: pr1_path_W p q ▸ (Wtype_eq p q)..2 = q :=
!sup_path_W..2
definition eta_path_W (p : w = w') : Wtype_eq (p..1) (p..2) = p :=
by cases p; cases w; exact idp
definition transport_pr1_path_W {B' : A → Type} (p : w.1 = w'.1) (q : p ▸ w.2 = w'.2)
: transport (λx, B' x.1) (Wtype_eq p q) = transport B' p :=
by cases w; cases w'; cases p; cases q; exact idp
definition path_W_uncurried (pq : Σ(p : w.1 = w'.1), p ▸ w.2 = w'.2) : w = w' :=
by cases pq with p q; exact (Wtype_eq p q)
definition sup_path_W_uncurried (pq : Σ(p : w.1 = w'.1), p ▸ w.2 = w'.2)
: ⟨(path_W_uncurried pq)..1, (path_W_uncurried pq)..2⟩ = pq :=
by cases pq with p q; exact (sup_path_W p q)
definition pr1_path_W_uncurried (pq : Σ(p : w.1 = w'.1), p ▸ w.2 = w'.2)
: (path_W_uncurried pq)..1 = pq.1 :=
!sup_path_W_uncurried..1
definition pr2_path_W_uncurried (pq : Σ(p : w.1 = w'.1), p ▸ w.2 = w'.2)
: (pr1_path_W_uncurried pq) ▸ (path_W_uncurried pq)..2 = pq.2 :=
!sup_path_W_uncurried..2
definition eta_path_W_uncurried (p : w = w') : path_W_uncurried ⟨p..1, p..2⟩ = p :=
!eta_path_W
definition transport_pr1_path_W_uncurried {B' : A → Type} (pq : Σ(p : w.1 = w'.1), p ▸ w.2 = w'.2)
: transport (λx, B' x.1) (@path_W_uncurried A B w w' pq) = transport B' pq.1 :=
by cases pq with p q; exact (transport_pr1_path_W p q)
definition isequiv_path_W /-[instance]-/ (w w' : W a, B a)
: is_equiv (@path_W_uncurried A B w w') :=
adjointify path_W_uncurried
(λp, ⟨p..1, p..2⟩)
eta_path_W_uncurried
sup_path_W_uncurried
definition equiv_path_W (w w' : W a, B a) : (Σ(p : w.1 = w'.1), p ▸ w.2 = w'.2) ≃ (w = w') :=
equiv.mk path_W_uncurried !isequiv_path_W
definition double_induction_on {P : (W a, B a) → (W a, B a) → Type} (w w' : W a, B a)
(H : ∀ (a a' : A) (f : B a → W a, B a) (f' : B a' → W a, B a),
(∀ (b : B a) (b' : B a'), P (f b) (f' b')) → P (sup a f) (sup a' f')) : P w w' :=
begin
revert w',
induction w with a f IH,
intro w',
cases w' with a' f',
apply H, intro b b',
apply IH
end
/- truncatedness -/
open is_trunc
definition trunc_W [instance] (n : trunc_index)
[HA : is_trunc (n.+1) A] : is_trunc (n.+1) (W a, B a) :=
begin
fapply is_trunc_succ_intro, intro w w',
eapply (double_induction_on w w'), intro a a' f f' IH,
fapply is_trunc_equiv_closed,
{ apply equiv_path_W},
{ fapply is_trunc_sigma,
intro p, cases p, esimp,
apply pi.is_trunc_eq_pi}
end
end Wtype