(* (c) Copyright 2006-2016 Microsoft Corporation and Inria.                  *)
(* Distributed under the terms of CeCILL-B.                                  *)
From mathcomp
From mathcomp
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(* This file provide support for intervals in numerical and real domains.    *)
(* The datatype (interval R) gives a formal characterization of an           *)
(* interval, as the pair of its right and left bounds.                       *)
(*    interval R    == the type of formal intervals on R.                    *)
(*    x \in i       == when i is a formal interval on a numeric domain,      *)
(*                   \in can be used to test membership.                     *)
(*    itvP x_in_i   == where x_in_i has type x \in i, if i is ground,        *)
(*                   gives a set of rewrite rules that x_in_i imply.         *)
(* x <= y ?< if c   == x is smaller than y, and strictly if c is true        *)
(*                                                                           *)
(* We provide a set of notations to write intervals (see below)              *)
(* `[a, b], `]a, b], ..., `]-oo, a], ..., `]-oo, +oo[                        *)
(* We also provide the lemma subitvP which computes the inequalities one     *)
(* needs to prove when trying to prove the inclusion of intervals.           *)
(*                                                                           *)
(* Remark that we cannot implement a boolean comparison test for intervals   *)
(* on an arbitrary numeric domains, for this problem might be undecidable.   *)
(* Note also that type (interval R) may contain several inhabitants coding   *)
(* for the same interval. However, this pathological issues do nor arise     *)
(* when R is a real domain: we could provide a specific theory for this      *)
(* important case.                                                           *)
(*                                                                           *)
(* See also ``Formal proofs in real algebraic geometry: from ordered fields  *)
(* to quantifier elimination'', LMCS journal, 2012                           *)
(* by Cyril Cohen and Assia Mahboubi                                         *)
(*                                                                           *)
(* And "Formalized algebraic numbers: construction and first-order theory"   *)
(* Cyril Cohen, PhD, 2012, section 4.3.                                      *)
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Set Implicit Arguments.
Unset Strict Implicit.
Unset Printing Implicit Defensive.
Local Open Scope ring_scope.
Local Notation mid x y := ((x + y) / 2%:R).
Section LersifPo.
Variable R : numDomainType.
Definition lersif (x y : R) b := if b then x < y else x <= y.
Local Notation "x <= y ?< 'if' b" := (lersif x y b)
(at level 70, y at next level,
format "x '[hv'  <=  y '/'  ?<  'if'  b ']'") : ring_scope.

Lemma lersif_imply b1 b2 r1 r2 :
b2 ==> b1 -> r1 <= r2 ?< if b1 -> r1 <= r2 ?< if b2.