Function basket_tail 

fn basket_tail<R: Scalar>(
    grid: &[(f64, f64)],
    thresholds: &[Option<R>],
    n: usize,
    conditional: impl Fn(R, f64) -> R,
    reweight: Option<&dyn Fn(f64) -> R>,
) -> R

Σ wᵢ·P(N < n | mᵢ) over a factor grid, the conditional count distribution by the Poisson-binomial recursion truncated at n.