for (E element : elements) {
      // Explicit null check because TreeSet wrongly accepts add(null) when empty.
      set.add(checkNotNull(element));
    }
    return set;
  }

  public static class ContiguousSetGenerator extends AbstractContiguousSetGenerator {
    @Override
    protected SortedSet<Integer> create(Integer[] elements) {
      return checkedCreate(nullCheckedTreeSet(elements));
    }
  }

    for (E element : elements) {
      // Explicit null check because TreeSet wrongly accepts add(null) when empty.
      set.add(checkNotNull(element));
    }
    return set;
  }

  public static class ContiguousSetGenerator extends AbstractContiguousSetGenerator {
    @Override
    protected SortedSet<Integer> create(Integer[] elements) {
      return checkedCreate(nullCheckedTreeSet(elements));
    }
  }