(** * Powers of 2 *)

  (** For the powers of two, that will be widely used, a more direct
      calculus is possible. [shift n m] computes [2^n * m], i.e.
      [m] shifted by [n] positions *)

  Definition shift_nat (n:nat) (z:positive) := nat_rect _ z (fun _ => xO) n.Definition shift_pos (n z:positive) := Pos.iter xO z n.Definition shift (n:Z) (z:positive) :=
    match n with
      | Z0 => z
      | Zpos p => Pos.iter xO z p
      | Zneg p => z
    end.Definition two_power_nat (n:nat) := Zpos (shift_nat n 1).Definition two_power_pos (x:positive) := Zpos (shift_pos x 1).Definition two_p (x:Z) :=
    match x with
      | Z0 => 1
      | Zpos y => two_power_pos y
      | Zneg y => 0
    end.

Lemma two_p_S x : 0 <= x -> two_p (Z.succ x) = 2 * two_p x.