78 lines
2.7 KiB
Text
78 lines
2.7 KiB
Text
/-
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Copyright (c) 2015 Floris van Doorn. All rights reserved.
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Released under Apache 2.0 license as described in the file LICENSE.
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Module: hit.quotient
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Authors: Floris van Doorn
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Declaration of set-quotients
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-/
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import .type_quotient .trunc
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open eq is_trunc trunc type_quotient
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namespace quotient
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section
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parameters {A : Type} (R : A → A → hprop)
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-- set-quotients are just truncations of type-quotients
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definition quotient : Type := trunc 0 (type_quotient (λa a', trunctype.carrier (R a a')))
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definition class_of (a : A) : quotient :=
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tr (class_of _ a)
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definition eq_of_rel {a a' : A} (H : R a a') : class_of a = class_of a' :=
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ap tr (eq_of_rel _ H)
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theorem is_hset_quotient : is_hset quotient :=
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begin unfold quotient, exact _ end
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protected definition rec {P : quotient → Type} [Pt : Πaa, is_hset (P aa)]
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(Pc : Π(a : A), P (class_of a)) (Pp : Π⦃a a' : A⦄ (H : R a a'), eq_of_rel H ▸ Pc a = Pc a')
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(x : quotient) : P x :=
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begin
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apply (@trunc.rec_on _ _ P x),
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{ intro x', apply Pt},
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{ intro y, fapply (type_quotient.rec_on y),
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{ exact Pc},
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{ intros,
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apply concat, apply transport_compose; apply Pp}}
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end
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protected definition rec_on [reducible] {P : quotient → Type} (x : quotient)
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[Pt : Πaa, is_hset (P aa)] (Pc : Π(a : A), P (class_of a))
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(Pp : Π⦃a a' : A⦄ (H : R a a'), eq_of_rel H ▸ Pc a = Pc a') : P x :=
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rec Pc Pp x
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theorem rec_eq_of_rel {P : quotient → Type} [Pt : Πaa, is_hset (P aa)]
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(Pc : Π(a : A), P (class_of a)) (Pp : Π⦃a a' : A⦄ (H : R a a'), eq_of_rel H ▸ Pc a = Pc a')
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{a a' : A} (H : R a a') : apd (rec Pc Pp) (eq_of_rel H) = Pp H :=
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!is_hset.elim
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protected definition elim {P : Type} [Pt : is_hset P] (Pc : A → P)
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(Pp : Π⦃a a' : A⦄ (H : R a a'), Pc a = Pc a') (x : quotient) : P :=
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rec Pc (λa a' H, !tr_constant ⬝ Pp H) x
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protected definition elim_on [reducible] {P : Type} (x : quotient) [Pt : is_hset P]
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(Pc : A → P) (Pp : Π⦃a a' : A⦄ (H : R a a'), Pc a = Pc a') : P :=
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elim Pc Pp x
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theorem elim_eq_of_rel {P : Type} [Pt : is_hset P] (Pc : A → P)
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(Pp : Π⦃a a' : A⦄ (H : R a a'), Pc a = Pc a') {a a' : A} (H : R a a')
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: ap (elim Pc Pp) (eq_of_rel H) = Pp H :=
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begin
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apply (@cancel_left _ _ _ _ (tr_constant (eq_of_rel H) (elim Pc Pp (class_of a)))),
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rewrite [-apd_eq_tr_constant_con_ap,↑elim,rec_eq_of_rel],
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end
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/-
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there are no theorems to eliminate to the universe here,
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because the universe is generally not a set
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-/
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end
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end quotient
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attribute quotient.class_of [constructor]
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attribute quotient.rec quotient.elim [unfold-c 7]
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attribute quotient.rec_on quotient.elim_on [unfold-c 4]
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