def each_col
      if block_given?
        @col_keys.each_key { |c| yield(c, col(c)) }
        self
      else
        Enumerator.new do |y|
          @col_keys.each_key { |c| y << [c, col(c)] }
        end
      end
    end
def each_col
      if block_given?
        @col_keys.each_key { |c| yield(c, col(c)) }
        self
      else
        Enumerator.new do |y|
          @col_keys.each_key { |c| y << [c, col(c)] }
        end
      end
    end
Iterate by col