Function save_merged_by 

pub fn save_merged_by<T: Serialize + DeserializeOwned>(
    path: &Path,
    read: &Read,
    state: &T,
    settle: impl FnOnce(&mut Table, Option<&Table>),
) -> Result<(), String>

The same, settle then given the merged table and the file as it was just before (none when missing): for what a field-by-field merge cannot say (a mark that must never go backwards).