Section round_plus_eq_zero_aux.
Variable rnd : R -> Z.
Context { valid_rnd : Valid_rnd rnd }.

Lemma round_plus_neq_0_aux :
forall x y,
(cexp beta fexp x <= cexp beta fexp y)%Z ->
format x -> format y ->
(0 < x + y)%R ->
round beta fexp rnd (x + y) <> 0%R.