code: def get_main_domain(self, website):
          
          subdomains = website['subdomains']
          main_domains = set()
          for sub in subdomains:
              for d in self._domains:
                  if sub == d or sub.endswith("." + d):
                      main_domains.add(d)
          if len(main_domains) > 1:
              logger.error(
                  "The secure site %s cover multiple domains, it should be splitted" % website['name']
              )
              exit(1)
          elif not main_domains:
              logger.error("We cannot find the main domain for %s" % website['name'])
          return list(main_domains)[0]
docs: Given a list of subdomains, return the main domain of them
              If the subdomain are across multiple domain, then we cannot have a single website
              it should be splitted