remove method
Implementation
ORSet<T> remove(T element) {
if (!_addSet.containsKey(element)) return this;
final newRemove = Map<T, Set<String>>.from(_removeSet.map((k, v) => MapEntry(k, Set<String>.from(v))));
newRemove[element] = (newRemove[element] ?? {})..addAll(_addSet[element]!);
return ORSet(addSet: _addSet, removeSet: newRemove);
}