input: Write a python function to crawl a web page and collect all unique links on the page.
  assert crawl('http://example.com') == {'http://example.com/link1', 'http://example.com/link2'}
  assert crawl('http://example.org') == {'http://example.org/page1', 'http://example.org/page2', 'http://example.org/page3'}
  assert crawl('http://example.net') == {'http://example.net/home', 'http://example.net/about'}
output: def crawl(url):
      import requests
      from bs4 import BeautifulSoup
      from urllib.parse import urljoin
  
      response = requests.get(url)
      soup = BeautifulSoup(response.text, 'html.parser')
      links = set()
  
      for a_tag in soup.find_all('a', href=True):
          full_url = urljoin(url, a_tag['href'])
          links.add(full_url)
  
      return links