}

    @VisibleForTesting
    void forceJdk() {
      requireNonNull(impl); // see the comment on the field
      this.impl = new JdkBackedSetBuilderImpl<>(impl);
    }

    final void copyIfNecessary() {
      if (forceCopy) {
        copy();
        forceCopy = false;
      }
    }

    void copy() {
      requireNonNull(impl); // see the comment on the field
      impl = impl.copy();

     * @throws NullPointerException if {@code element} is null
     */
    @CanIgnoreReturnValue
    @Override
    public Builder<E> add(E element) {
      checkNotNull(element);
      copyIfNecessary();
      if (n == elements.length) {
        sortAndDedup();
        /*
         * sortAndDedup may have made enough room for this element, but that's not necessarily good