Definition Bsign x :=
  match x with
  | B754_nan s _ => s
  | B754_zero s => s
  | B754_infinity s => s
  | B754_finite s _ _ _ => s
  end.Definition sign_FF x :=
  match x with
  | F754_nan s _ => s
  | F754_zero s => s
  | F754_infinity s => s
  | F754_finite s _ _ => s
  end.

Theorem Bsign_FF2B : forall x H, Bsign (FF2B x H) = sign_FF x.