Function explain_zero 

pub fn explain_zero(
    root: &Path,
    config: &IndexConfig,
    open: &OpenIndex,
    filter: &QueryFilter,
    pattern: &str,
    substring_only: Option<usize>,
    excluded_scoped: usize,
    bracket_rewritten: bool,
) -> Option<ZeroHint>

The first rule that applies to this zero result; None when none does.