Definition x_ (i : nat) := exist (in_Z_ (2 ^ w)) (x / 2 ^ (Z_of_nat i * w) mod 2 ^ w) (mod_in_Z__ (x / 2 ^ (Z_of_nat i * w))).Definition f' (d_Sir_Si : Z_ (2 ^ w) * Z_ (2 ^ w)) (xi : Z_ (2 ^ w)) :=
  (d_i M_in_Z_ ptM'cM'' d_Sir_Si xi, r_i M_in_Z_ ptM'cM'' d_Sir_Si xi).Fixpoint foldlz (f : Z_ (2 ^ w) * Z_ (2 ^ w) -> Z_ (2 ^ w) -> Z_ (2 ^ w) * Z_ (2 ^ w)) (d_Sir_Si : Z_ (2 ^ w) * Z_ (2 ^ w)) (a:nat) {struct a} : 
 Z_ (2 ^ w) * Z_ (2 ^ w) :=
  match a with
  | O   => d_Sir_Si
  | S i => foldlz f (f d_Sir_Si (x_ i)) i
  end.

Lemma P_O : 0 <= x -> x < 2 ^ (Z_of_nat O * w) -> P M p (fst (foldlz f' (zero_in_Z_, zero_in_Z_) O)) (snd (foldlz f' (zero_in_Z_, zero_in_Z_) O)) (x / 2 ^ w) (x_ 0).