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(* 02110-1301 USA                                                     *)
(* Contribution to the Coq Library   V6.3 (July 1999)                    *)
(****************************************************************************)
(*                 The Calculus of Inductive Constructions                  *)
(*                                                                          *)
(*                                Projet Coq                                *)
(*                                                                          *)
(*                     INRIA                        ENS-CNRS                *)
(*              Rocquencourt                        Lyon                    *)
(*                                                                          *)
(*                                Coq V5.10                                 *)
(*                              Nov 25th 1994                               *)
(*                                                                          *)
(****************************************************************************)
(*                               Ensf_inter.v                               *)
(****************************************************************************)
(*              Formal Language Theory                                      *)
(*                                                                          *)
(*              Judicael Courant - Jean-Christophe Filliatre                *)
(*                                                                          *)
(*              Developped in V5.8  June-July 1993                          *)
(*              Ported     to V5.10 October 1994                            *)
(****************************************************************************)
(*									*)
(*  INTERSECTION 							*)
(*   L'intersection de 2 ensembles est definie comme un predicat sur    *)
(*   3 ensembles A B C en disant que C est l'intresection de A et B     *)
(*   si C est le plus grand ensemble inclus dans A et dans B		*)
(*									*)
Definition inter (A B C : Ensf) : Prop :=
inclus C A /\
inclus C B /\ (forall x : Elt, dans x A -> dans x B -> dans x C).
unfold inter in |- *.
intros.
elim H0; clear H0.
intros H0 H1; elim H1; clear H1; intros H1 H2.
elim H; clear H.
intros H3 H4; elim H4; clear H4; intros H4 H5.
split; auto.
split.
apply empty_inclus.
intros.
cut (dans x b \/ dans x c); auto.
intro H7; elim H7; auto.
Qed.
unfold inter in |- *.
intros.
elim H0; clear H0.
intros H0 H1; elim H1; clear H1; intros H1 H2.
elim H; clear H.
intros H3 H4; elim H4; clear H4; intros H4 H5.
split; auto.
split; auto.
intros.
cut (dans x A \/ dans x B); auto.
intro H7; elim H7; auto.
Qed.
unfold inter in |- *.
intros.
elim H; clear H; intros H Ht; elim Ht; clear Ht; intros H1 H2.
red in |- *; intro.
cut (dans x empty); auto.
intro.
apply dans_empty_imp_P with x; auto.
Qed.

Lemma sym_inter : forall A B C : Ensf, inter A B C -> inter B A C.