251 lines
9.7 KiB
Text
251 lines
9.7 KiB
Text
/-
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Copyright (c) 2015 Jakob von Raumer. All rights reserved.
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Released under Apache 2.0 license as described in the file LICENSE.
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Authors: Floris van Doorn
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Properties of is_trunc and trunctype
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-/
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-- NOTE: the fact that (is_trunc n A) is a mere proposition is proved in .hprop_trunc
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import types.pi types.eq types.equiv ..function
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open eq sigma sigma.ops pi function equiv is_trunc.trunctype
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is_equiv prod is_trunc.trunc_index pointed nat
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namespace is_trunc
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variables {A B : Type} {n : trunc_index}
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/- theorems about trunctype -/
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protected definition trunctype.sigma_char.{l} (n : trunc_index) :
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(trunctype.{l} n) ≃ (Σ (A : Type.{l}), is_trunc n A) :=
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begin
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fapply equiv.MK,
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{ intro A, exact (⟨carrier A, struct A⟩)},
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{ intro S, exact (trunctype.mk S.1 S.2)},
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{ intro S, induction S with S1 S2, reflexivity},
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{ intro A, induction A with A1 A2, reflexivity},
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end
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definition trunctype_eq_equiv (n : trunc_index) (A B : n-Type) :
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(A = B) ≃ (carrier A = carrier B) :=
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calc
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(A = B) ≃ (to_fun (trunctype.sigma_char n) A = to_fun (trunctype.sigma_char n) B)
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: eq_equiv_fn_eq_of_equiv
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... ≃ ((to_fun (trunctype.sigma_char n) A).1 = (to_fun (trunctype.sigma_char n) B).1)
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: equiv.symm (!equiv_subtype)
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... ≃ (carrier A = carrier B) : equiv.refl
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theorem is_trunc_is_embedding_closed (f : A → B) [Hf : is_embedding f] [HB : is_trunc n B]
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(Hn : -1 ≤ n) : is_trunc n A :=
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begin
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induction n with n,
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{exact !empty.elim Hn},
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{apply is_trunc_succ_intro, intro a a',
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fapply @is_trunc_is_equiv_closed_rev _ _ n (ap f)}
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end
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theorem is_trunc_is_retraction_closed (f : A → B) [Hf : is_retraction f]
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(n : trunc_index) [HA : is_trunc n A] : is_trunc n B :=
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begin
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revert A B f Hf HA,
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induction n with n IH,
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{ intro A B f Hf HA, induction Hf with g ε, fapply is_contr.mk,
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{ exact f (center A)},
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{ intro b, apply concat,
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{ apply (ap f), exact (center_eq (g b))},
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{ apply ε}}},
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{ intro A B f Hf HA, induction Hf with g ε,
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apply is_trunc_succ_intro, intro b b',
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fapply (IH (g b = g b')),
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{ intro q, exact ((ε b)⁻¹ ⬝ ap f q ⬝ ε b')},
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{ apply (is_retraction.mk (ap g)),
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{ intro p, induction p, {rewrite [↑ap, con.left_inv]}}},
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{ apply is_trunc_eq}}
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end
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definition is_embedding_to_fun (A B : Type) : is_embedding (@to_fun A B) :=
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λf f', !is_equiv_ap_to_fun
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theorem is_trunc_trunctype [instance] (n : trunc_index) : is_trunc n.+1 (n-Type) :=
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begin
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apply is_trunc_succ_intro, intro X Y,
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fapply is_trunc_equiv_closed,
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{apply equiv.symm, apply trunctype_eq_equiv},
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fapply is_trunc_equiv_closed,
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{apply equiv.symm, apply eq_equiv_equiv},
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induction n,
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{apply @is_contr_of_inhabited_hprop,
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{apply is_trunc_is_embedding_closed,
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{apply is_embedding_to_fun} ,
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{exact unit.star}},
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{apply equiv_of_is_contr_of_is_contr}},
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{apply is_trunc_is_embedding_closed,
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{apply is_embedding_to_fun},
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{exact unit.star}}
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end
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/- theorems about decidable equality and axiom K -/
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theorem is_hset_of_axiom_K {A : Type} (K : Π{a : A} (p : a = a), p = idp) : is_hset A :=
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is_hset.mk _ (λa b p q, eq.rec_on q K p)
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theorem is_hset_of_relation.{u} {A : Type.{u}} (R : A → A → Type.{u})
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(mere : Π(a b : A), is_hprop (R a b)) (refl : Π(a : A), R a a)
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(imp : Π{a b : A}, R a b → a = b) : is_hset A :=
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is_hset_of_axiom_K
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(λa p,
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have H2 : transport (λx, R a x → a = x) p (@imp a a) = @imp a a, from !apd,
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have H3 : Π(r : R a a), transport (λx, a = x) p (imp r)
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= imp (transport (λx, R a x) p r), from
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to_fun (equiv.symm !heq_pi) H2,
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have H4 : imp (refl a) ⬝ p = imp (refl a), from
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calc
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imp (refl a) ⬝ p = transport (λx, a = x) p (imp (refl a)) : transport_eq_r
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... = imp (transport (λx, R a x) p (refl a)) : H3
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... = imp (refl a) : is_hprop.elim,
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cancel_left H4)
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definition relation_equiv_eq {A : Type} (R : A → A → Type)
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(mere : Π(a b : A), is_hprop (R a b)) (refl : Π(a : A), R a a)
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(imp : Π{a b : A}, R a b → a = b) (a b : A) : R a b ≃ a = b :=
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@equiv_of_is_hprop _ _ _
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(@is_trunc_eq _ _ (is_hset_of_relation R mere refl @imp) a b)
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imp
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(λp, p ▸ refl a)
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local attribute not [reducible]
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theorem is_hset_of_double_neg_elim {A : Type} (H : Π(a b : A), ¬¬a = b → a = b)
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: is_hset A :=
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is_hset_of_relation (λa b, ¬¬a = b) _ (λa n, n idp) H
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section
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open decidable
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--this is proven differently in init.hedberg
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theorem is_hset_of_decidable_eq (A : Type) [H : decidable_eq A] : is_hset A :=
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is_hset_of_double_neg_elim (λa b, by_contradiction)
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end
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theorem is_trunc_of_axiom_K_of_leq {A : Type} (n : trunc_index) (H : -1 ≤ n)
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(K : Π(a : A), is_trunc n (a = a)) : is_trunc (n.+1) A :=
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@is_trunc_succ_intro _ _ (λa b, is_trunc_of_imp_is_trunc_of_leq H (λp, eq.rec_on p !K))
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theorem is_trunc_succ_of_is_trunc_loop (Hn : -1 ≤ n) (Hp : Π(a : A), is_trunc n (a = a))
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: is_trunc (n.+1) A :=
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begin
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apply is_trunc_succ_intro, intros a a',
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apply is_trunc_of_imp_is_trunc_of_leq Hn, intro p,
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induction p, apply Hp
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end
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theorem is_trunc_succ_iff_is_trunc_loop (A : Type) (Hn : -1 ≤ n) :
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is_trunc (n.+1) A ↔ Π(a : A), is_trunc n (a = a) :=
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iff.intro _ (is_trunc_succ_of_is_trunc_loop Hn)
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theorem is_trunc_iff_is_contr_loop_succ (n : ℕ) (A : Type)
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: is_trunc n A ↔ Π(a : A), is_contr (Ω[succ n](Pointed.mk a)) :=
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begin
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revert A, induction n with n IH,
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{ intros, esimp [Iterated_loop_space], apply iff.intro,
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{ intros H a, apply is_contr.mk idp, apply is_hprop.elim},
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{ intro H, apply is_hset_of_axiom_K, intros, apply is_hprop.elim}},
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{ intros, transitivity _, apply @is_trunc_succ_iff_is_trunc_loop n, constructor,
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apply iff.pi_iff_pi, intros,
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transitivity _, apply IH,
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assert H : Πp : a = a, Ω(Pointed.mk p) = Ω(Pointed.mk (idpath a)),
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{ intros, fapply Pointed_eq,
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{ esimp, transitivity _,
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apply eq_equiv_fn_eq_of_equiv (equiv_eq_closed_right _ p⁻¹),
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esimp, apply eq_equiv_eq_closed, apply con.right_inv, apply con.right_inv},
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{ esimp, apply con.left_inv}},
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transitivity _,
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apply iff.pi_iff_pi, intro p,
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rewrite [↑Iterated_loop_space,H],
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apply iff.refl,
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apply iff.imp_iff, reflexivity}
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end
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theorem is_trunc_iff_is_contr_loop (n : ℕ) (A : Type)
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: is_trunc (n.-2.+1) A ↔ (Π(a : A), is_contr (Ω[n](pointed.Mk a))) :=
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begin
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induction n with n,
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{ esimp [sub_two,Iterated_loop_space], apply iff.intro,
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intro H a, exact is_contr_of_inhabited_hprop a,
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intro H, apply is_hprop_of_imp_is_contr, exact H},
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{ apply is_trunc_iff_is_contr_loop_succ},
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end
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end is_trunc open is_trunc
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namespace trunc
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variable {A : Type}
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protected definition code (n : trunc_index) (aa aa' : trunc n.+1 A) : n-Type :=
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trunc.rec_on aa (λa, trunc.rec_on aa' (λa', trunctype.mk' n (trunc n (a = a'))))
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protected definition encode (n : trunc_index) (aa aa' : trunc n.+1 A) : aa = aa' → trunc.code n aa aa' :=
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begin
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intro p, induction p, apply (trunc.rec_on aa),
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intro a, esimp [trunc.code,trunc.rec_on], exact (tr idp)
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end
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protected definition decode (n : trunc_index) (aa aa' : trunc n.+1 A) : trunc.code n aa aa' → aa = aa' :=
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begin
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eapply (trunc.rec_on aa'), eapply (trunc.rec_on aa),
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intro a a' x, esimp [trunc.code, trunc.rec_on] at x,
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apply (trunc.rec_on x), intro p, exact (ap tr p)
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end
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definition trunc_eq_equiv [constructor] (n : trunc_index) (aa aa' : trunc n.+1 A)
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: aa = aa' ≃ trunc.code n aa aa' :=
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begin
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fapply equiv.MK,
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{ apply trunc.encode},
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{ apply trunc.decode},
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{ eapply (trunc.rec_on aa'), eapply (trunc.rec_on aa),
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intro a a' x, esimp [trunc.code, trunc.rec_on] at x,
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refine (@trunc.rec_on n _ _ x _ _),
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intro x, apply is_trunc_eq,
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intro p, induction p, reflexivity},
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{ intro p, induction p, apply (trunc.rec_on aa), intro a, exact idp},
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end
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definition tr_eq_tr_equiv [constructor] (n : trunc_index) (a a' : A)
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: (tr a = tr a' :> trunc n.+1 A) ≃ trunc n (a = a') :=
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!trunc_eq_equiv
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definition is_trunc_trunc_of_is_trunc [instance] [priority 500] (A : Type)
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(n m : trunc_index) [H : is_trunc n A] : is_trunc n (trunc m A) :=
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begin
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revert A m H, eapply (trunc_index.rec_on n),
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{ clear n, intro A m H, apply is_contr_equiv_closed,
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{ apply equiv.symm, apply trunc_equiv, apply (@is_trunc_of_leq _ -2), exact unit.star} },
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{ clear n, intro n IH A m H, induction m with m,
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{ apply (@is_trunc_of_leq _ -2), exact unit.star},
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{ apply is_trunc_succ_intro, intro aa aa',
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apply (@trunc.rec_on _ _ _ aa (λy, !is_trunc_succ_of_is_hprop)),
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eapply (@trunc.rec_on _ _ _ aa' (λy, !is_trunc_succ_of_is_hprop)),
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intro a a', apply (is_trunc_equiv_closed_rev),
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{ apply tr_eq_tr_equiv},
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{ exact (IH _ _ _)}}}
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end
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open equiv.ops
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definition unique_choice {P : A → Type} [H : Πa, is_hprop (P a)] (f : Πa, ∥ P a ∥) (a : A)
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: P a :=
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!trunc_equiv (f a)
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end trunc open trunc
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namespace function
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variables {A B : Type}
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definition is_surjective_of_is_equiv [instance] (f : A → B) [H : is_equiv f] : is_surjective f :=
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λb, !center
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definition is_equiv_equiv_is_embedding_times_is_surjective [constructor] (f : A → B)
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: is_equiv f ≃ (is_embedding f × is_surjective f) :=
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equiv_of_is_hprop (λH, (_, _))
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(λP, prod.rec_on P (λH₁ H₂, !is_equiv_of_is_surjective_of_is_embedding))
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end function
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