(*****************************************************)
(**                Division by 2                     *)
Definition half (x : N.t) := snd (half_aux x).
#[global]
Instance half_aux_wd : Proper (N.eq ==> N.eq*N.eq) half_aux.
#[global]
Instance half_wd : Proper (N.eq==>N.eq) half.

Theorem half_nz : forall n, 1 < n -> 0 < half n.