input: Write a python function to create an authentication system using Flask and CAS. 
  assert authenticate_user('valid_ticket') == 'You are authorized!'
  assert authenticate_user('invalid_ticket') == 'You are not authorized!'
  assert authenticate_user('') == 'You are not authorized!'
output: from flask import Flask, request
  import requests
  
  app = Flask(__name__)
  
  CAS_VERIFY_URL = 'https://your-cas-server/cas/validate'
  
  def authenticate_user(ticket):
      if not ticket:
          return 'You are not authorized!'
      
      params = {'ticket': ticket, 'service': 'http://your-service-url'}
      response = requests.get(CAS_VERIFY_URL, params=params)
      
      if response.status_code == 200 and 'yes' in response.text:
          return 'You are authorized!'
      else:
          return 'You are not authorized!'
  
  @app.route('/login')
  def login():
      ticket = request.args.get('ticket')
      return authenticate_user(ticket)