input: from typing import Set
  
  def crawl(url: str) -> Set[str]:
      """Crawl a given webpage and extract all unique external links found on the page and linked pages.
      The crawler starts from the given url and continues until there are no more unique links to follow.
      
      >>> crawl('http://example.com')
      {'http://example.com/page1', 'http://example.com/page2', ...}
      """
      visited = set()
      queue = [url]
      
      while queue:
          curr = queue.pop(0)
          visited.add(curr)
          response = requests.get(curr)
          
          if response.ok:
              soup = BeautifulSoup(response.text, 'html.parser')
              
              # Scrape the page here if needed
              
              # Find all the links
              links = soup.find_all('a', href=re.compile('^http'))
              
              for link in links:
                  if link['href'] not in visited:
                      queue.append(link['href'])
      
      return visited
output:     visited = set()
      queue = [url]
      
      while queue:
          curr = queue.pop(0)
          if curr in visited:
              continue
          visited.add(curr)
          try:
              response = requests.get(curr)
              if response.ok:
                  soup = BeautifulSoup(response.text, 'html.parser')
                  links = soup.find_all('a', href=re.compile('^http'))
                  for link in links:
                      if link['href'] not in visited:
                          queue.append(link['href'])
          except requests.RequestException:
              continue
      
      return visited