fn next_opposite(
line: LineNumber,
opposites: &HashMap<LineNumber, HashSet<LineNumber>, BuildHasherDefault<FxHasher>>,
prev_opposite: Option<LineNumber>
) -> Option<LineNumber>
fn next_opposite(
line: LineNumber,
opposites: &HashMap<LineNumber, HashSet<LineNumber>, BuildHasherDefault<FxHasher>>,
prev_opposite: Option<LineNumber>
) -> Option<LineNumber>