Proof with auto with typeclass_instances.
intros rnd Zrnd x.
destruct (total_order_T x 0) as [[Hx|Hx]|Hx].
rewrite <- (Ropp_involutive x).
destruct (round_DN_or_UP rnd (- - x)) as [Hr|Hr] ; rewrite Hr.
rewrite round_DN_opp.
apply generic_format_opp.
apply generic_format_round_pos...
now apply Ropp_0_gt_lt_contravar.
rewrite round_UP_opp.
apply generic_format_opp.
apply generic_format_round_pos...
now apply Ropp_0_gt_lt_contravar.
rewrite Hx.
rewrite round_0...
apply generic_format_0.
now apply generic_format_round_pos.
Qed.

Theorem round_DN_pt :
forall x,
Rnd_DN_pt generic_format x (round Zfloor x).