Method operator +=

unittests/catch.hpp:5531–5534

        auto end() const -> iterator { return { *this, iterator::EndTag() }; }

        auto operator += ( Column const& col ) -> Columns& {
            m_columns.push_back( col );
            return *this;
        }
        auto operator + ( Column const& col ) -> Columns {
            Columns combined = *this;
            combined += col;