Function mnl_logsum 

pub fn mnl_logsum(costs: &[f64], theta: f64) -> f64

Log-sum (inclusive value) of a set of costs under MNL.

$$\text{logsum} = -\frac{1}{\theta} \ln \sum_k \exp(-\theta \cdot C_k)$$

This is the expected minimum cost (“accessibility” or “consumer surplus”) of the choice set.