Proof with auto with typeclass_instances.
intros x y He Hx Hy Hp Hxy.
destruct (Req_dec (x + y) 0) as [H0|H0].
exact H0.
destruct (ln_beta beta (x + y)) as (exy, Hexy).
simpl.
specialize (Hexy H0).
destruct (Zle_or_lt exy (fexp exy)) as [He'|He'].
(* . *)
assert (H: (x + y)%R = F2R (Float beta (Ztrunc (x * bpow (- fexp exy)) +
Ztrunc (y * bpow (- fexp exy))) (fexp exy))).
rewrite (subnormal_exponent beta fexp exy x He' Hx) at 1.
rewrite (subnormal_exponent beta fexp exy y He' Hy) at 1.
now rewrite <- F2R_plus, Fplus_same_exp.
rewrite H in Hxy.
rewrite round_generic in Hxy...
now rewrite <- H in Hxy.
apply generic_format_F2R.
intros _.
rewrite <- H.
unfold canonic_exp.
rewrite ln_beta_unique with (1 := Hexy).
apply Zle_refl.
(* . *)
elim Rle_not_lt with (1 := round_le beta _ rnd _ _ (proj1 Hexy)).
rewrite (Rabs_pos_eq _ Hp).
rewrite Hxy.
rewrite round_generic...
apply bpow_gt_0.
apply generic_format_bpow.
apply Zlt_succ_le.
now rewrite (Zsucc_pred exy) in He'.
Qed.
End round_plus_eq_zero_aux.
Variable rnd : R -> Z.
Context { valid_rnd : Valid_rnd rnd }.

(** rnd(x+y)=0 -> x+y = 0 provided this is not a FTZ format *)
Theorem round_plus_eq_zero :
forall x y,
format x -> format y ->
round beta fexp rnd (x + y) = 0%R ->
(x + y = 0)%R.