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