intros n x; case x; unfold Fshift, Fdigit, is_Fzero in |- *; simpl in |- *.
intros p1 p2 H; apply digitAdd; auto.
Qed.

Theorem FshiftCorrect : forall (n : nat) (x : float), Fshift n x = x :>R.
intros n x; unfold FtoR in |- *; simpl in |- *.