docs: def cookieDomainForRequest(self, request):
          """
          Pick a domain to use when setting cookies.
  
          @type request: L{nevow.inevow.IRequest}
          @param request: Request to determine cookie domain for
  
          @rtype: C{str} or C{None}
          @return: Domain name to use when setting cookies, or C{None} to
              indicate that only the domain in the request should be used
          """
          host = request.getHeader('host')
          if host is None:
              # This is a malformed request that we cannot possibly handle
              # safely, fall back to the default behaviour.
              return None
  
          host = host.split(':')[0]
          for domain in self._domains:
              suffix = "." + domain
              if host == domain:
                  # The request is for a domain which is directly recognized.
                  if self._enableSubdomains:
                      # Subdomains are enabled, so the suffix is returned to
                      # enable the cookie for this domain and all its subdomains.
                      return suffix
  
                  # Subdomains are not enabled, so None is returned to allow the
                  # default restriction, which will enable this cookie only for
                  # the domain in the request, to apply.
                  return None
  
              if self._enableSubdomains and host.endswith(suffix):
                  # The request is for a subdomain of a directly recognized
                  # domain and subdomains are enabled.  Drop the unrecognized
                  # subdomain portion and return the suffix to enable the cookie
                  # for this domain and all its subdomains.
                  return suffix
  
          if self._enableSubdomains:
              # No directly recognized domain matched the request.  If subdomains
              # are enabled, prefix the request domain with "." to make the
              # cookie valid for that domain and all its subdomains.  This
              # probably isn't extremely useful.  Perhaps it shouldn't work this
              # way.
              return "." + host
  
          # Subdomains are disabled and the domain from the request was not
          # recognized.  Return None to get the default behavior.
          return None