Definition MSB (x : float) := Zpred (Z_of_nat (Fdigit radix x) + Fexp x).

Theorem MSB_mix : forall x y : float, ~ is_Fzero x -> ~ is_Fzero y -> (Fabs x * Float 1%nat (MSB y) < radix * (Fabs y * Float 1%nat (MSB x)))%R.