Proof with auto with typeclass_instances.
intros x y Fx Fy H.
apply generic_format_FLT_FIX...
rewrite Zplus_comm; assumption.
apply generic_format_FIX_FLT, FIX_format_generic in Fx.
apply generic_format_FIX_FLT, FIX_format_generic in Fy.
destruct Fx as (nx,(H1x,H2x)).
destruct Fy as (ny,(H1y,H2y)).
apply generic_format_FIX.
exists (Float beta (Fnum nx+Fnum ny)%Z emin).
split;[idtac|reflexivity].
rewrite H1x,H1y; unfold F2R; simpl.
rewrite H2x, H2y.
rewrite Z2R_plus; ring.
Qed.
End Fprop_plus_FLT.
Section Fprop_plus_mult_ulp.
Variable beta : radix.
Notation bpow e := (bpow beta e).
Variable fexp : Z -> Z.
Context { valid_exp : Valid_exp fexp }.
Context { monotone_exp : Monotone_exp fexp }.
Variable rnd : R -> Z.
Context { valid_rnd : Valid_rnd rnd }.
Notation format := (generic_format beta fexp).
Notation cexp := (canonic_exp beta fexp).

(* Specifications: *)
Lemma ex_shift :
forall x e, format x -> (e <= cexp x)%Z ->
exists m, (x = Z2R m * bpow e)%R.