Section Double_round_plus.

Lemma mag_minus_separated :
forall fexp : Z -> Z, Valid_exp fexp ->
forall x y,
0 < x -> 0 < y -> y < x ->
bpow (mag x - 1) < x ->
generic_format beta fexp x -> (mag y <= fexp (mag x))%Z ->
(mag (x - y) = mag x :> Z).