(** 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.