pub fn generate_document<'a>(
    doc_id: Option<i64>,
    rng: &mut SmallRng,
    schema: &Schema,
    title_prefix: &'a str,
    title_power: usize,
    body_prefix: &'a str,
    body_power: usize,
    tag_prefix: &'a str,
    tag_power: usize
) -> SummaDocument<'a>