intros; unfold MSB, Fshift, Fdigit in |- *; simpl in |- *.
rewrite digitAdd; auto with zarith.
rewrite inj_plus; unfold Zpred in |- *; ring.
Qed.

Theorem MSB_comp :
forall (x y : float) (n : nat), ~ is_Fzero x -> x = y :>R -> MSB x = MSB y.