def add_class(class_name)
      @fragment.children.each do |c|
        c['class'] = ((c['class']||'').split(' ') + [class_name]).join(' ')
      end
    end
def add_class(class_name)
      @fragment.children.each do |c|
        c['class'] = ((c['class']||'').split(' ') + [class_name]).join(' ')
      end
    end
Add a class to the children of this fragment