cut (Fdigit radix x <> 0%Z :>Z); unfold Zpred in |- *;
auto with zarith.
unfold Fdigit in |- *.
red in |- *; intros H'0; absurd (digit radix (Fnum x) = 0); auto with zarith.
apply not_eq_sym; apply lt_O_neq; apply digitNotZero; auto.
Qed.

Theorem MSB_le_abs :
forall x : float, ~ is_Fzero x -> (Float 1%nat (MSB x) <= Fabs x)%R.