Section Cosets.
Variables (gT : finGroupType) (Q A : {set gT}).
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(* Cosets are right cosets of elements in the normaliser.                     *)
(* We let cosets coerce to GroupSet.sort, so they inherit the group subset    *)
(* base group structure. Later we will define a proper group structure on     *)
(* cosets, which will then hide the inherited structure once coset_of unifies *)
(* with FinGroup.sort; the coercion to GroupSet.sort will no longer be used.  *)
(*   Note that for Hx Hy : coset_of H, Hx * Hy : {set gT} can mean either     *)
(*    set_of_coset (mulg Hx Hy) OR mulg (set_of_coset Hx) (set_of_coset Hy).  *)
(* However, since the two terms are actually convertible, we can live with    *)
(* this ambiguity.                                                            *)
(*   We take great care that neither the type coset_of H, nor its Canonical   *)
(* finGroupType structure, nor the coset H morphism depend on the actual      *)
(* group structure of H. Otherwise, rewriting would be extremely awkward      *)
(* because all our equalities are stated at the set level.                    *)
(*   The trick we use is to interpret coset_of A, when A is any set, as the   *)
(* type of cosets of the group <<A>> generated by A, in the group A <*> N(A)  *)
(* generated by A and its normaliser. This coincides with the type of         *)
(* bilateral cosets of A when A is a group. We restrict the domain of coset A *)
(* to 'N(A), so that we get almost all the same conversion equalities as if   *)
(* we had forced A to be a group in the first place; the only exception, that *)
(* 1 : coset_of A : {set gT} = <<A>> rather than A, can be handled by genGid. *)
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Notation H := <<A>>.
Definition coset_range := [pred B in rcosets H 'N(A)].
Record coset_of : Type :=
Coset { set_of_coset :> GroupSet.sort gT; _ : coset_range set_of_coset }.
Canonical coset_subType := Eval hnf in [subType for set_of_coset].
Definition coset_eqMixin := Eval hnf in [eqMixin of coset_of by <:].
Canonical coset_eqType := Eval hnf in EqType coset_of coset_eqMixin.
Definition coset_choiceMixin := [choiceMixin of coset_of by <:].
Canonical coset_choiceType := Eval hnf in ChoiceType coset_of coset_choiceMixin.
Definition coset_countMixin := [countMixin of coset_of by <:].
Canonical coset_countType := Eval hnf in CountType coset_of coset_countMixin.
Canonical coset_subCountType := Eval hnf in [subCountType of coset_of].
Definition coset_finMixin := [finMixin of coset_of by <:].
Canonical coset_finType := Eval hnf in FinType coset_of coset_finMixin.
Canonical coset_subFinType := Eval hnf in [subFinType of coset_of].
(* We build a new (canonical) structure of groupType for cosets.              *)
(* When A is a group, this is the largest possible quotient 'N(A) / A.        *)
Definition coset_mul B C := Coset (coset_range_mul B C).
Definition coset_inv B := Coset (coset_range_inv B).
Definition coset_of_groupMixin :=
FinGroup.Mixin coset_mulP coset_oneP coset_invP.
Canonical coset_baseGroupType :=
Eval hnf in BaseFinGroupType coset_of coset_of_groupMixin.
Canonical coset_groupType := FinGroupType coset_invP.
(* Projection of the initial group type over the cosets groupType.  *)
Definition coset x : coset_of := insubd (1 : coset_of) (H :* x).
Canonical coset_morphism := Morphism coset_morphM.
Implicit Type xbar : coset_of.
(* coset is an inverse to repr *)

Lemma card_quotient A : A \subset 'N(H) -> #|A / H| = #|A : H|.