(* This program is free software; you can redistribute it and/or      *)
(* modify it under the terms of the GNU Lesser General Public License *)
(* as published by the Free Software Foundation; either version 2.1   *)
(* of the License, or (at your option) any later version.             *)
(*                                                                    *)
(* This program is distributed in the hope that it will be useful,    *)
(* but WITHOUT ANY WARRANTY; without even the implied warranty of     *)
(* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the      *)
(* GNU General Public License for more details.                       *)
(*                                                                    *)
(* You should have received a copy of the GNU Lesser General Public   *)
(* License along with this program; if not, write to the Free         *)
(* Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA *)
(* 02110-1301 USA                                                     *)
Set Implicit Arguments.
Unset Strict Implicit.
Require Export Sgroup_facts.
Require Export Parts.
Section Def.
Variable G : SGROUP.
Section Sub_sgroup.
Variable H : part_set G.
Hypothesis
Hprop :
forall x y : G,
in_part x H -> in_part y H -> in_part (sgroup_law _ x y) H.
Definition subsgroup_law : law_of_composition H.
unfold law_of_composition in |- *.
apply
(Build_Map
(A:=cart (set_of_subtype_image (part H)) (set_of_subtype_image (part H)))
(B:=H)
(Ap:=fun
x : cart (set_of_subtype_image (part H))
(set_of_subtype_image (part H)) =>
Build_subtype
(Hprop (subtype_prf (proj1 x)) (subtype_prf (proj2 x))))).
red in |- *.
simpl in |- *.
unfold cart_eq, subtype_image_equal in |- *.
simpl in |- *.
unfold cart_eq, subtype_image_equal in |- *.
intuition.
Defined.
Definition subsgroup_sgroup : sgroup.
apply (Build_sgroup (sgroup_set:=H)).
apply (Build_sgroup_on (E:=H) (sgroup_law_map:=subsgroup_law)).
red in |- *.
simpl in |- *.
unfold subtype_image_equal in |- *.
simpl in |- *.
auto with algebra.
Defined.
End Sub_sgroup.
Record subsgroup : Type :=
{subsgroup_part : Predicate G;
subsgroup_prop :
forall x y : G,
in_part x subsgroup_part ->
in_part y subsgroup_part -> in_part (sgroup_law _ x y) subsgroup_part}.
Definition sgroup_of_subsgroup (H : subsgroup) :=
subsgroup_sgroup (subsgroup_prop (s:=H)).
End Def.
Coercion sgroup_of_subsgroup : subsgroup >-> sgroup.
Coercion subsgroup_part : subsgroup >-> Predicate.
Section Injection.
Variable G : SGROUP.
Variable H : subsgroup G.

Lemma subsgroup_in_prop :
forall x y : G, in_part x H -> in_part y H -> in_part (sgroup_law _ x y) H.