/// the original `RegionVid` to an scc index. In some cases, we
/// may have multiple `R1` values mapping to the same `R2` key -- that
/// is ok, the two sets will be merged.
/// the original `RegionVid` to an scc index. In some cases, we
/// may have multiple `R1` values mapping to the same `R2` key -- that
/// is ok, the two sets will be merged.