intros n m; case n; simpl in |- *; auto with zarith; intros; red in |- *;
discriminate.
Qed.
Hint Resolve Fabs_Fzero: float.

Theorem Fdigit_abs : forall x : float, Fdigit radix (Fabs x) = Fdigit radix x.
intros x; unfold Fabs, Fdigit in |- *; simpl in |- *.