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(*         *   The Coq Proof Assistant / The Coq Development Team       *)
(*  v      *   INRIA, CNRS and contributors - Copyright 1999-2018       *)
(* <O___,, *       (see CREDITS file for the list of authors)           *)
(*   \VV/  **************************************************************)
(*    //   *    This file is distributed under the terms of the         *)
(*         *     GNU Lesser General Public License Version 2.1          *)
(*         *     (see LICENSE file for the text of the license)         *)
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Require Export Omega.
Local Open Scope Z_scope.
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(** About parity *)
Notation two_or_two_plus_one := Z_modulo_2 (only parsing).
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(** The biggest power of 2 that is stricly less than [a]  Easy to compute: replace all "1" of the binary representation by "0", except the first "1" (or the first one :-) *)
Fixpoint floor_pos (a:positive) : positive :=
match a with
| xH => 1%positive
| xO a' => xO (floor_pos a')
| xI b' => xO (floor_pos b')
end.
Definition floor (a:positive) := Zpos (floor_pos a).
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(** Two more induction principles over [Z]. *)

Theorem Z_lt_abs_rec :
forall P:Z -> Set,
(forall n:Z, (forall m:Z, Z.abs m < Z.abs n -> P m) -> P n) ->
forall n:Z, P n.