def <<(element)
      TextLine.send :assert_element_type, element

      return self unless ELEMENT_TYPES.include? element.type
      return self unless element.options[:location] == @location

      value = element.value

      if element.type == :smart_quote
        appent_quote(value)
      else
        append_text(value)
      end

      self
    end
def <<(element)
      TextLine.send :assert_element_type, element

      return self unless ELEMENT_TYPES.include? element.type
      return self unless element.options[:location] == @location

      value = element.value

      if element.type == :smart_quote
        appent_quote(value)
      else
        append_text(value)
      end

      self
    end
Append Kramdown::Element's content to this element.
 @param element [Kramdown::Element]
 @raise ArgumentError if element is in different location.