Function pool_base 

pub fn pool_base(s: f64, c: &Challenge) -> f64

A pooled lesson’s score without its usefulness factor: s / (0.5 + mean belief) = relevance × squash(A) × trust / NEUTRAL (rank, memory.rs Memory::weight). Under another belief the score is base × (0.5 + its mean), and propensity orders by base × (0.5 + draw), so borg debug ope (ope.rs) can replay a slot under other weights.