nth_child (HTML::Selector)

    # File lib/action_controller/vendor/html-scanner/html/selector.rb, line 722
        def nth_child(a, b, of_type, reverse)
          # a = 0 means select at index b, if b = 0 nothing selected
          return lambda { |element| false } if a == 0 && b == 0
          # a < 0 and b < 0 will never match against an index
          return lambda { |element| false } if a < 0 && b < 0
          b = a + b + 1 if b < 0   # b < 0 just picks last element from each group
          b -= 1 unless b == 0  # b == 0 is same as b == 1, otherwise zero based
          lambda do |element|
            # Element must be inside parent element.
            return false unless element.parent && element.parent.tag?
            index = 0
            # Get siblings, reverse if counting from last.
            siblings = element.parent.children
            siblings = siblings.reverse if reverse
            # Match element name if of-type, otherwise ignore name.
            name = of_type ? element.name : nil
            found = false
            for child in siblings
              # Skip text nodes/comments.
              if child.tag? && (name == nil || child.name == name)
                if a == 0
                  # Shortcut when a == 0 no need to go past count
                  if index == b
                    found = child.equal?(element)
                    break
                  end
                elsif a < 0
                  # Only look for first b elements
                  break if index > b
                  if child.equal?(element)
                    found = (index % a) == 0
                    break
                  end
                else
                  # Otherwise, break if child found and count ==  an+b
                  if child.equal?(element)
                    found = (index % a) == b
                    break
                  end
                end
                index += 1
              end
            end
            found
          end
        end