sclass WebScraper {
  S baseURL;
  new Set<S> urlsSeen;
  new LinkedHashSet<S> linksToFollow;
  
  *(S *baseURL, S startURL) {
    addLink(startURL);
  }
  
  void addLinks(Iterable<S> urls) { for (S url : urls) addLink(url); }
  void addLink(S url) {
    if (!urlsSeen.contains(url) && startsWith(url, baseURL))
      linksToFollow.add(url);
  }
  
  bool step {
    if (empty(linksToFollow)) false;
    _loadURL(popFirst(linksToFollow));
    true;
  }
  
  void _loadURL(S url) {
    urlsSeen.add(url);
    addLinks(pairsA(webScraper_getLinks(url)));
  }
  
  run {
    while (step())
      print("URLs checked: " + l(urlsSeen));
    print("Scraping done.");
  }
}