Definition Fdigit (p : float) := digit radix (Fnum p).Definition Fshift (n : nat) (x : float) :=
  Float (Fnum x * Zpower_nat radix n) (Fexp x - n).

Theorem FshiftAdd : forall (n m : nat) (p : float), Fshift (n + m) p = Fshift n (Fshift m p).