Function build_labels_count_distribution_matrix 

pub fn build_labels_count_distribution_matrix(
    cover: &mut Cover,
    candidates: &[usize],
) -> Vec<Vec<Vec<usize>>>

Class counts for every pair of candidates: matrix[i][j] counts the instances in the right branch of both candidates[i] and candidates[j], and matrix[i][i] those in the right branch of candidates[i]. Every other leaf of a depth-2 tree follows by subtraction.