Function fold_error 

fn fold_error(
    table: &Table,
    model: &Model,
    x: &[f32],
    width: usize,
    words: &[Vec<String>],
    ids: &[usize],
    messages: &[Message],
) -> Result<f64, LearnError>

The largest difference between a table’s score and its model’s on some messages (x, their standardized rows); over 1e-4, the fold is broken.