    # File lib/action_controller/vendor/html-scanner/html/selector.rb, line 772
        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