Function early_propensity 

pub fn early_propensity(
    pool: &[(f64, &Challenge)],
    session: &str,
    topics: &[String],
) -> HashMap<String, f64>

Each pooled lesson’s chance of being delivered early under the session’s posterior (early over early_pool): propensity’s draws, keeping in each the first EARLY_CAP and of them what render_early shows for topics. Exactly 1 for every lesson when the whole pool always shows.