Function score_pattern_flat_coverage 

pub fn score_pattern_flat_coverage(
    layout: &NaryPatternBatchLayout,
    pattern_index: usize,
    candidates: &[FlatRelation],
    positives: &[Vec<u64>],
    negatives: &[Vec<u64>],
) -> (u32, u32)

Coverage counts for one pattern over example sets, via the flat walk.