End GroupActionTheory.
Notation "''C_' ( | to ) ( A )" := (gacent_group to A) : Group_scope.
Notation "''C_' ( G | to ) ( A )" :=
(setI_group G 'C_(|to)(A)) : Group_scope.
Notation "''C_' ( | to ) [ a ]" := (gacent_group to [set a%g]) : Group_scope.
Notation "''C_' ( G | to ) [ a ]" :=
(setI_group G 'C_(|to)[a]) : Group_scope.
Notation "to \ sAD" := (ract_groupAction to sAD) : groupAction_scope.
Notation "<[ nGA ] >" := (actby_groupAction nGA) : groupAction_scope.
Notation "to / H" := (quotient_groupAction to H) : groupAction_scope.
Notation "to %% H" := (mod_groupAction to H) : groupAction_scope.
Notation "to \o f" := (comp_groupAction to f) : groupAction_scope.
(* Operator group isomorphism. *)
Section MorphAction.
Variables (aT1 aT2 : finGroupType) (rT1 rT2 : finType).
Variables (D1 : {group aT1}) (D2 : {group aT2}).
Variables (to1 : action D1 rT1) (to2 : action D2 rT2).
Variables (A : {set aT1}) (R S : {set rT1}).
Variables (h : rT1 -> rT2) (f : {morphism D1 >-> aT2}).
Hypotheses (actsDR : {acts D1, on R | to1}) (injh : {in R &, injective h}).
Hypothesis defD2 : f @* D1 = D2.
Hypotheses (sSR : S \subset R) (sAD1 : A \subset D1).
Hypothesis hfJ : {in S & D1, morph_act to1 to2 h f}.

Lemma morph_astab : f @* 'C(S | to1) = 'C(h @: S | to2).