(* Copyright (C) 2005-2008 Sebastien Briais *)
(* http://lamp.epfl.ch/~sbriais/ *)
(* This library 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 library 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 *)
(* Lesser General Public License for more details. *)
(* You should have received a copy of the GNU Lesser General Public *)
(* License along with this library; if not, write to the Free Software *)
(* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA *)
(* 02110-1301 USA *)
Require Export List Permutation.
(** we define some notions on lists such as being a permutation of *)
(** insertion x l l' iff l' is l where x has been inserted somewhere *)
Inductive insertion (A:Set) : A -> list A -> list A -> Prop :=
head_insertion : forall (x:A)(l:list A),(insertion A x l (x::l))
|tail_insertion : forall (x y:A)(l l':list A),(insertion A x l l')->(insertion A x (y::l) (y::l')).
induction xs.
simpl.
apply head_insertion.
simpl.
apply tail_insertion.
auto.
Qed.
intros.
induction H;simpl;tauto.
Qed.
induction l;simpl;try tauto;intros.
inversion H;simpl;try tauto.
case H0;try tauto.
right;auto.
Qed.
induction l;simpl;try tauto;intros.
case H;intro.
rewrite H0;exists l;apply head_insertion.
elim (IHl H0);intro l';intro.
exists (a::l');apply tail_insertion;trivial.
Qed.
intros.
induction H;simpl in H0.
case H0;intro H1;try (symmetry in H1);tauto.
case H0;simpl;intro;tauto.
Qed.
(** a list is a set iff all the elements are pairwise distinct *)
Inductive is_set (A:Set) : list A->Prop :=
nil_is_set : (is_set A nil)
|cons_is_set : forall (x:A)(l:list A),(is_set A l)->~(In x l)->(is_set A (x::l)).
induction 1;intros.
inversion H;trivial.
inversion H0.
apply cons_is_set.
apply IHinsertion;trivial.
intro;apply H4;apply (insertion_inclusion A x l l');trivial.
Qed.
induction l;simpl;try tauto;intros.
inversion H;rewrite <- H3 in H0;inversion H0.
simpl in H7;trivial.
intro.
case H10;intro.
apply H9;rewrite H11;eapply insertion_in;apply H5.
elim (IHl l'0 x H5 H8 H11).
Qed.
induction 1.
intros.
inversion H0.
apply cons_is_set.
apply nil_is_set.
auto.
intros.
inversion H2.
subst x1.
subst l0.
subst l'.
apply cons_is_set.
eapply IHis_set.
apply H0.
apply head_insertion.
auto.
subst x1;subst y;subst l0.
apply cons_is_set.
eapply IHis_set with x0;auto.
red;intro.
apply H1.
simpl.
right;auto.
red;intro.
case (in_insertion_inv _ _ _ _ _ H7 H3);intro.
apply H1.
simpl.
left;auto.
apply H0.
auto.
Qed.
(** l' is a permutation of l *)
Inductive is_permutation (A:Set) : list A->list A->Prop :=
nil_is_permutation : (is_permutation A nil nil)
|cons_is_permutation : forall (l l':list A),(is_permutation A l l')->forall (x:A)(l'':(list A)),(insertion A x l' l'')->(is_permutation A (x::l) l'').
induction l;intros;simpl in H.
destruct l'.
apply nil_is_permutation.
elim (H a);intros.
elim H3;simpl;tauto.
inversion H0.
symmetry in H2;rewrite H2 in H;elim (H a);intros.
rewrite H2 in H6;rewrite H2.
assert (In x l');auto.
elim (in_insertion A x l' H8).
intro l'';intro.
apply cons_is_permutation with l'';trivial.
apply IHl;trivial.
split;intro.
elim (H x0);intros.
elim (in_insertion_inv A x0 x l'' l');auto.
intro;rewrite H13 in H10;rewrite H2 in H5;tauto.
elim (H x0);intros.
case H12;try tauto.
apply (insertion_inclusion A x l'' l');trivial.
intro;rewrite <- H13 in H10.
elim (is_set_insertion_in A l'' l' x);trivial.
eapply is_set_insertion;eauto.
Qed.
induction l.
apply nil_is_permutation.
eapply cons_is_permutation;[apply IHl | apply head_insertion].
Qed.

(** if l' is l where x has been inserted then l' is a permutation of x::l *)
Lemma insertion_is_permutation : forall (A:Set)(l l':list A)(x:A),(insertion A x l l')->(is_permutation A (x::l) l').