def each_row
      if block_given?
        @row_keys.each_key { |r| yield(r, row(r)) }
        self
      else
        Enumerator.new do |y|
          @row_keys.each_key { |r| y << [r, row(r)] }
        end
      end
    end
def each_row
      if block_given?
        @row_keys.each_key { |r| yield(r, row(r)) }
        self
      else
        Enumerator.new do |y|
          @row_keys.each_key { |r| y << [r, row(r)] }
        end
      end
    end
Iterate by row