Function train 

pub(crate) fn train(
    s: &Session,
    dirs: &Dirs,
    ask: &TrainAsk,
    progress: &mut dyn FnMut(&Progress),
    cancel: &Cancel,
) -> Result<Report, String>

A training, the corpus brought up to date first when asked: a trial unless replace; its summary, and its test’s detail (lines when errors are asked).