Function operator-

Source/external/json.hpp:13430–13435

! @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