input: Write a function to crawl web pages and extract all hyperlinks.
  assert crawler("https://en.wikipedia.org/wiki/Web_crawling")[:5] == ['/wiki/Web_indexing', '/wiki/Internet_bot', '/wiki/Robots.txt', '/wiki/Web_scraping', '/wiki/Web_archiving']
  assert crawler("https://en.wikipedia.org/wiki/Python_(programming_language)")[:5] == ['/wiki/Programming_language', '/wiki/Interpreted_language', '/wiki/High-level_programming_language', '/wiki/Guido_van_Rossum', '/wiki/ABC_(programming_language)']
  assert crawler("https://en.wikipedia.org/wiki/Data_science")[:5] == ['/wiki/Information_science', '/wiki/Knowledge_extraction', '/wiki/Knowledge_discovery', '/wiki/Big_data', '/wiki/Computer_science']
output: import requests
  from bs4 import BeautifulSoup
  
  def crawler(url):
      response = requests.get(url)
      soup = BeautifulSoup(response.content, 'html.parser')
      links = [a['href'] for a in soup.find_all('a', href=True) if a['href'].startswith('/wiki/')]
      return links