only_child

# File actionview/lib/action_view/vendor/html-scanner/html/selector.rb, line 774
    def only_child(of_type)
      lambda do |element|
        # Element must be inside parent element.
        return false unless element.parent && element.parent.tag?
        name = of_type ? element.name : nil
        other = false
        for child in element.parent.children
          # Skip text nodes/comments.
          if child.tag? && (name == nil || child.name == name)
            unless child.equal?(element)
              other = true
              break
            end
          end
        end
        !other
      end
    end