Definition double_round_plus_hyp fexp1 fexp2 :=
  (forall ex ey, (fexp1 (ex + 1) - 1 <= ey)%Z -> (fexp2 ex <= fexp1 ey)%Z)
  /\ (forall ex ey, (fexp1 (ex - 1) + 1 <= ey)%Z -> (fexp2 ex <= fexp1 ey)%Z)
  /\ (forall ex ey, (fexp1 ex - 1 <= ey)%Z -> (fexp2 ex <= fexp1 ey)%Z)
  /\ (forall ex ey, (ex - 1 <= ey)%Z -> (fexp2 ex <= fexp1 ey)%Z).

Lemma double_round_minus : forall (fexp1 fexp2 : Z -> Z), Valid_exp fexp1 -> Valid_exp fexp2 -> forall (choice1 choice2 : Z -> bool), double_round_plus_hyp fexp1 fexp2 -> forall x y, generic_format beta fexp1 x -> generic_format beta fexp1 y -> double_round_eq fexp1 fexp2 choice1 choice2 (x - y).