input: Write a python function to crawl multiple URLs and extract data using XPath expression.
  assert crawl("http://example.com") == ['expected_data_from_example.com']
  assert crawl("http://anotherexample.com") == ['expected_data_from_anotherexample.com']
  assert crawl("http://yetanotherexample.com") == ['expected_data_from_yetanotherexample.com']
output: def crawl(url):
      import requests
      from lxml import html
      
      response = requests.get(url)
      tree = html.fromstring(response.content)
      
      # Example XPath expression to extract data. This should be customized based on the actual data needed from each URL.
      data = tree.xpath('//your_xpath_expression_here')
      
      return data