(* <KEEP TOGETHER - plusminustimesuhwm *)
Definition plusw (x y : Z_ (2 ^ w)) :=
  exist (in_Z_ (2 ^ w)) ((x + y) mod 2 ^ w)
             (mod_in_Z__ (x + y)).Definition minusw (x y : Z_ (2 ^ w)) :=
  exist (in_Z_ (2 ^ w)) ((x - y) mod 2 ^ w) (mod_in_Z__ (x - y)).Definition multw (x y : Z_ (2 ^ w)) :=
  exist (in_Z_ (2 ^ w)) (x * y mod 2 ^ w) (mod_in_Z__ (x * y)).Definition uhwm (x y : Z_ (2 ^ w)) :=
  exist (in_Z_ (2 ^ w)) (x * y / 2 ^ w) (is_in_Z_R_uhwm x y).(* KEEP TOGETHER> - plusminustimesuhwm *)

(* <KEEP TOGETHER - shifts *)
Definition multp2 (x : Z_ (2 ^ w)) (i : Z_ w) := 
  exist (in_Z_ (2 ^ w)) (2 ^ i * x mod 2 ^ w) (mod_in_Z__ (2 ^ i * x)).Definition ursh (x : Z_ (2 ^ w)) (i : Z_ w) := 
  exist (in_Z_ (2 ^ w)) (x / 2 ^ i) (is_in_Z_R_ursh x i).(* KEEP TOGETHER> - shifts *)

(* <KEEP TOGETHER - ltw *)
Definition ltw (x y t e : Z_ (2 ^ w)) :=
  If (Zlt_bool x y) t e.

Lemma Z_from_multw : forall x y : Z_ (2 ^ w), _Z (multw x y) = x * y mod 2 ^ w.