Method operator ++

unittests/catch.hpp:5513–5519

                return row;
            }
            auto operator ++() -> iterator& {
                for( size_t i = 0; i < m_columns.size(); ++i ) {
                    if (m_iterators[i] != m_columns[i].end())
                        ++m_iterators[i];
                }
                return *this;
            }
            auto operator ++(int) -> iterator {
                iterator prev( *this );
                operator++();