input: from typing import Set, List
  
  def crawl(url: str) -> Set[str]:
      """A web crawler that starts from a given URL and visits all linked pages,
      scraping the content from each visited page. The crawler only visits URLs
      that start with 'http' and keeps track of visited URLs using a set.
      >>> crawl('http://example.com')
      {'http://example.com', 'http://example.com/page1', 'http://example.com/page2'}
      """
      visited = set()
      queue = [url]
  
      while queue:
          curr = queue.pop(0)
          if curr in visited:
              continue
          visited.add(curr)
          response = requests.get(curr)
  
          if response.ok:
              soup = BeautifulSoup(response.text, 'html.parser')
  
              # scrape the page here
  
              # find all the links
              links = soup.find_all('a', href=re.compile('^http'))
  
              for link in links:
                  href = link.get('href')
                  if href and href not in visited:
                      queue.append(href)
      return visited
output:     visited = set()
      queue = [url]
  
      while queue:
          curr = queue.pop(0)
          if curr in visited:
              continue
          visited.add(curr)
          response = requests.get(curr)
  
          if response.ok:
              soup = BeautifulSoup(response.text, 'html.parser')
  
              # scrape the page here
  
              # find all the links
              links = soup.find_all('a', href=re.compile('^http'))
  
              for link in links:
                  href = link.get('href')
                  if href and href not in visited:
                      queue.append(href)
      return visited