Definition pick_variety (s : SS.elt) (st : STt) :=
  STexists_ (fun t => pick_variety_aux s t) st.Definition exists_witness (f : SS.elt -> bool) (s : SS.t) : option SS.elt :=
  SS.choose (SS.filter f s).Definition pick_varieties_aux (s1 : SS.elt) (ss : SS.t) (st : STt)
                              : (option (SS.elt * SS.elt)) :=
  match ((exists_witness (fun s2 => let i := S.inter s1 s2 in
                                    pick_variety i st)) ss) with
    | None => None
    | Some s2 => Some(s1,s2)
  end.Definition pick_varieties (ss : SS.t) (st : STt)
                          : (option (SS.elt * SS.elt)) :=
  match (exists_witness (fun s =>
                           match (pick_varieties_aux s (SS.remove s ss) st) with
                             | None => false
                             | _ => true
                           end) ss) with
    | None => None
    | Some s1 => pick_varieties_aux s1 (SS.remove s1 ss) st
  end.Definition eqop (p1 p2 : option SS.elt) :=
  match p1,p2 with
    | None, None => True
    | Some s1, Some s2 => True
    | _, _ => False
  end.Definition eqopp (p1 p2 : option (SS.elt * SS.elt)) :=
  match p1,p2 with
    | None, None => True
    | Some s1, Some s2 => True
    | _, _ => False
  end.

Lemma pick_varieties_ok_2 : forall s1 s2 ss st, pick_varieties ss st = Some(s1,s2) -> SS.In s2 (SS.remove s1 ss).