code: public void append(Set<T> set) {
          if (set == null)
              throw new NullPointerException("set cannot be null");
          sets.add(set);
      }
docs: Adds the provided set to this {@code CombinedSet}.  Note that this does
  <i>not</i> perform an element-wise addition of the contents of {@code
  set}, but rather keeps a reference to {@code set} so that any changes to
  {@code set} will be reflected in this {@code CombinedSet}.