End Rmissing.
Section Z2R.
(** Z2R function (Z -> R) *)
Fixpoint P2R (p : positive) :=
match p with
| xH => 1%R
| xO xH => 2%R
| xO t => (2 * P2R t)%R
| xI xH => 3%R
| xI t => (1 + 2 * P2R t)%R
end.
Definition Z2R n :=
match n with
| Zpos p => P2R p
| Zneg p => Ropp (P2R p)
| Z0 => 0%R
end.

Theorem Z2R_plus :
forall m n, (Z2R (m + n) = Z2R m + Z2R n)%R.