Function apply_cross 

pub fn apply_cross(
    set: &mut [Candidate],
    name: &str,
    doc: CrossDoc,
    builds: &mut Vec<Candidate>,
) -> Vec<String>

One agent’s rulings onto the set. Builds are collected rather than pushed so a build cannot be ruled on in the round that produced it.

Returns what was dropped and why, for the caller to log.