Section Zround_opp.
Variable rnd : R -> Z.
Context { valid_rnd : Valid_rnd rnd }.
Definition Zrnd_opp x := Zopp (rnd (-x)).
Global Instance valid_rnd_opp : Valid_rnd Zrnd_opp.
Proof with auto with typeclass_instances.
split.
(* *)
intros x y Hxy.
unfold Zrnd_opp.
apply Zopp_le_cancel.
rewrite 2!Zopp_involutive.
apply Zrnd_le...
now apply Ropp_le_contravar.
(* *)
intros n.
unfold Zrnd_opp.
rewrite <- Z2R_opp, Zrnd_Z2R...
apply Zopp_involutive.
Qed.

Theorem round_opp :
forall x,
round rnd (- x) = Ropp (round Zrnd_opp x).