(* Subgroup Type construction. *)
(* We only expect to use this for abstract groups, so we don't project *)
(* the argument to a set.                                              *)
Definition sgval u := let: Subg x _ := u in x.
Canonical subg_subType := Eval hnf in [subType for sgval].
Definition subg_eqMixin := Eval hnf in [eqMixin of subg_of by <:].
Canonical subg_eqType := Eval hnf in EqType subg_of subg_eqMixin.
Definition subg_choiceMixin := [choiceMixin of subg_of by <:].
Canonical subg_choiceType := Eval hnf in ChoiceType subg_of subg_choiceMixin.
Definition subg_countMixin := [countMixin of subg_of by <:].
Canonical subg_countType := Eval hnf in CountType subg_of subg_countMixin.
Canonical subg_subCountType := Eval hnf in [subCountType of subg_of].
Definition subg_finMixin := [finMixin of subg_of by <:].
Canonical subg_finType := Eval hnf in FinType subg_of subg_finMixin.
Canonical subg_subFinType := Eval hnf in [subFinType of subg_of].

Lemma card_lcosets G H : #|lcosets H G| = #|G : H|.