def urls_linking_to(urls)
      unless urls.is_a?(Array)
        urls = [urls] unless urls.is_a?(Array)
        single = true
      end

      links = pages_linking_to(urls)
      links.each { |url, pages| links[url] = pages.map{|p| p.url} }

      if single and !links.empty?
        return links[urls.first]
      else
        return links
      end
    end
def urls_linking_to(urls)
      unless urls.is_a?(Array)
        urls = [urls] unless urls.is_a?(Array)
        single = true
      end

      links = pages_linking_to(urls)
      links.each { |url, pages| links[url] = pages.map{|p| p.url} }

      if single and !links.empty?
        return links[urls.first]
      else
        return links
      end
    end
If given a single URL (as a String or URI), returns an Array of URLs which link to that URL
 If given an Array of URLs, returns a Hash (URI => [URI, URI...]) of URLs linking to those URLs