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

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