add method
Implementation
ORSet<T> add(T element, String tag) {
final newAdd = Map<T, Set<String>>.from(_addSet.map((k, v) => MapEntry(k, Set<String>.from(v))));
newAdd[element] = (newAdd[element] ?? {})..add(tag);
return ORSet(addSet: newAdd, removeSet: _removeSet);
}