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(* 02110-1301 USA                                                     *)
(* Contribution to the Coq Library   V6.3 (July 1999)                    *)
(****************************************************************)
(*           The Calculus of Inductive Constructions            *)
(*                       COQ v5.10                              *)
(*                                                              *)
(* Laurent Arditi.  Laboratoire I3S. CNRS ura 1376.             *)
(* Universite de Nice - Sophia Antipolis                        *)
(* arditi@unice.fr, http://wwwi3s.unice.fr/~arditi/lolo.html    *)
(*                                                              *)
(* date: may 1995                                               *)
(* file: Lists_field.v                                          *)
(* contents: trunc, strip, sublist and elemlist for polymorphic *)
(* lists.                                                       *)
(****************************************************************)
Require Export Minus.
Require Export Lists_compl.
Section Lists_field.
Variable A : Set.
Fixpoint trunc (v : list A) : nat -> list A :=
fun n : nat =>
match v return (list A) with
| nil => nil
| b :: w =>
match n return (list A) with
| O => nil
| S p => b :: trunc w p
end
end.
Definition strip (v : list A) (n : nat) :=
rev A (trunc (rev A v) (length v - n)).
(* Selection d'une sous liste commencant a la position start, de longueur lengt *)
Definition sublist (v : list A) (start lengt : nat) :=
trunc (strip v start) lengt.
(* Selection de la liste contenant le i-eme element d'une liste *)
Definition elemlist (v : list A) (i : nat) := trunc (strip v i) 1.
(****************************************************************)
(* lemmes sur trunc *)

Lemma length_trunc :
forall (v : list A) (i : nat),
i <= length v -> length (trunc v i) = i.