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 FshiftFdigit : forall (n : nat) (x : float), ~ is_Fzero x -> Fdigit (Fshift n x) = Fdigit x + n.