Definition make_last {A} n (x def : A) :=
  Pos.peano_rect (fun _ => list A)
    (cons x nil)
    (fun _ F => cons def F) n.(* Replace the nth element of a list *)

Fixpoint list_replace l n b :=
match l with
| nil => make_last n b false
| cons a l =>
  Pos.peano_rect _
    (cons b l) (fun n _ => cons a (list_replace l n b)) n
end.Fixpoint boolean_witness p :=
match p with
| Cst c => nil
| Poly p i q =>
  if decide (null p) then
    let var := boolean_witness q in
    list_replace var i true
  else
    let var := boolean_witness p in
    list_replace var i false
end.

Lemma list_replace_nth_2 : forall var i x, list_nth i (list_replace var i x) false = x.