input: from typing import Set, List
  
  def crawl(url: str) -> Set[str]:
      """ Implements a web crawler that starts from a given url and scrapes the content 
      of pages by following the found links. It uses Depth First Search (DFS) for 
      traversing the pages. The function returns a set of visited URLs.
      >>> crawl('http://example.com')
      {'http://example.com', 'http://example.com/about', 'http://example.com/contact'}
      """
      visited = set()
      stack = [url]
      
      while stack:
          curr = stack.pop()
          if curr not in visited:
              visited.add(curr)
              response = requests.get(curr)
              
              if response.ok:
                  soup = BeautifulSoup(response.text, 'html.parser')
                  # scrape the page here 
                  links = soup.find_all('a', href=re.compile('^http'))
                  
                  for link in links:
                      if link['href'] not in visited:
                          stack.append(link['href'])
      
      return visited
output:     import requests
      from bs4 import BeautifulSoup
      import re
      
      visited = set()
      stack = [url]
      
      while stack:
          curr = stack.pop()
          if curr not in visited:
              visited.add(curr)
              response = requests.get(curr)
              
              if response.ok:
                  soup = BeautifulSoup(response.text, 'html.parser')
                  # scrape the page here 
                  links = soup.find_all('a', href=re.compile('^http'))
                  
                  for link in links:
                      if link['href'] not in visited:
                          stack.append(link['href'])
      
      return visited