docs: private void assign(HashMap<String, DBIDs> labelMap, String label, DBIDRef id) {
      if(labelMap.containsKey(label)) {
        DBIDs exist = labelMap.get(label);
        if(exist instanceof DBID) {
          ModifiableDBIDs n = DBIDUtil.newHashSet();
          n.add((DBID) exist);
          n.add(id);
          labelMap.put(label, n);
        }
        else {
          assert (exist instanceof HashSetModifiableDBIDs);
          assert (exist.size() > 1);
          ((ModifiableDBIDs) exist).add(id);
        }
      }
      else {
        labelMap.put(label, DBIDUtil.deref(id));
      }
    }