Function replacement 

pub(crate) fn replacement(
    new_ham_lost: u64,
    current_ham_lost: u64,
    compared: u64,
) -> (bool, &'static str)

Whether the new table replaces the one in place, and why, from the ham each sets aside among the compared test messages the one in place never learned from. None of them (nothing came since it was trained): nothing says it is better; the new one learned from the same mail, and replaces it.