code: public static <T> Set<T> unmodifiableSet(Set<? extends T> s) {
  		return Collections.unmodifiableSet(s);
  	}
docs: 返回包装后不可修改的Set.
  
  如果尝试修改，会抛出UnsupportedOperationException
  
  @see java.util.Collections#unmodifiableSet(Set)