(** This file is part of the Coquelicot formalization of real analysis in Coq: http://coquelicot.saclay.inria.fr/  Copyright (C) 2011-2015 Sylvie Boldo #<br /># Copyright (C) 2011-2015 Catherine Lelay #<br /># Copyright (C) 2011-2015 Guillaume Melquiond #<br /># Copyright (C) 2014 Xavier Onfroy  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 3 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 COPYING file for more details. *)
(** This file describes the definition and properties of the Henstock–Kurzweil (KH) integral. *)
Definition ith_step (ptd : @SF_seq R) i := nth 0 (SF_lx ptd) (S i) - nth 0 (SF_lx ptd) i.
Definition fine (delta : R -> posreal) ptd :=
forall i : nat, (i < SF_size ptd)%nat -> ith_step ptd i < delta (nth 0 (SF_ly ptd) i).
Definition KH_filter (P : SF_seq -> Prop) :=
exists delta, forall ptd, fine delta ptd -> P ptd.
Global Instance KH_filter_filter : Filter KH_filter.
Definition KH_fine (a b : R) :=
within (fun ptd => pointed_subdiv ptd /\ SF_h ptd = Rmin a b /\ last (SF_h ptd) (SF_lx ptd) = Rmax a b) KH_filter.

Lemma cousin :
forall a b delta, ~ ~ exists ptd,
pointed_subdiv ptd /\ fine delta ptd /\
SF_h ptd = Rmin a b /\ last (SF_h ptd) (SF_lx ptd) = Rmax a b.