Method operator-

lesson6-Segmentation/json.hpp:11315–11320

! @brief subtract from iterator @pre The iterator is initialized; i.e. `m_object != nullptr`. */

    @pre The iterator is initialized; i.e. `m_object != nullptr`.
    */
    iter_impl operator-(difference_type i) const
    {
        auto result = *this;
        result -= i;
        return result;
    }

    /*!
    @brief  return difference