// move on to next in r1
                    if (i1.hasNext()) {
                        res1 = i1.next();
                    } else {
                        done = true;
                    }
                }
            } else {
                // move on to next in r2
                if (i2.hasNext()) {
                    res2 = i2.next();
                } else {

   */
  public abstract @Nullable C previous(C value);

  /**
   * Returns a signed value indicating how many nested invocations of {@link #next} (if positive) or
   * {@link #previous} (if negative) are needed to reach {@code end} starting from {@code start}.
   * For example, if {@code end = next(next(next(start)))}, then {@code distance(start, end) == 3}
   * and {@code distance(end, start) == -3}. As well, {@code distance(a, a)} is always zero.
   *

        this.existPrePage = existPrePage;
    }

    /**
     * Check if the next page exists.
     * @return true if the next page exists.
     */
    public boolean isExistNextPage() {
        return existNextPage;
    }

    /**
     * Set if the next page exists.
     * @param existNextPage true if the next page exists.
     */
    public void setExistNextPage(final boolean existNextPage) {

    }

    /**
     * Checks if there is a next page.
     * @return True if a next page exists, false otherwise.
     */
    public boolean isExistNextPage() {
        return existNextPage;
    }

    /**
     * Sets the existence of a next page.
     * @param existNextPage True if a next page exists.
     */
    public void setExistNextPage(final boolean existNextPage) {

        this.existPrePage = existPrePage;
    }

    /**
     * Checks if next page exists.
     *
     * @return true if next page exists
     */
    public boolean isExistNextPage() {
        return existNextPage;
    }

    /**
     * Sets whether next page exists.
     *
     * @param existNextPage true if next page exists
     */
    public void setExistNextPage(final boolean existNextPage) {

        this.existPrePage = existPrePage;
    }

    /**
     * Checks if a next page exists.
     * @return true if a next page exists, false otherwise.
     */
    public boolean isExistNextPage() {
        return existNextPage;
    }

    /**
     * Sets whether a next page exists.
     * @param existNextPage true if a next page exists, false otherwise.
     */

    }

    /**
     * Checks if a next page exists.
     *
     * @return true if a next page exists, false otherwise.
     */
    public boolean isExistNextPage() {
        return existNextPage;
    }

    /**
     * Sets the flag for the existence of a next page.
     *
     * @param existNextPage true if a next page exists.
     */

        assertThat(it.hasNext(), is(true));
        assertThat(it.next(), is(sameClass(Integer.class)));

        assertThat(it.hasNext(), is(true));
        assertThat(it.next(), is(sameClass(Number.class)));

        assertThat(it.hasNext(), is(true));
        assertThat(it.next(), is(sameClass(Object.class)));

        assertThat(it.hasNext(), is(not(true)));
    }

    /**

    }

    /**
     * Checks if a next page exists.
     *
     * @return true if a next page exists, false otherwise
     */
    public boolean isExistNextPage() {
        return existNextPage;
    }

    /**
     * Sets whether a next page exists.
     *
     * @param existNextPage true if a next page exists, false otherwise
     */

    }

    /**
     * Returns true if a next page exists.
     *
     * @return True if a next page exists.
     */
    public boolean isExistNextPage() {
        return existNextPage;
    }

    /**
     * Sets whether a next page exists.
     *
     * @param existNextPage True if a next page exists.
     */
    public void setExistNextPage(final boolean existNextPage) {