docs: def _add(self, xer, primary, type):
          """
          Private method for adding a descriptor from the event loop.
  
          It takes care of adding it if  new or modifying it if already added
          for another state (read -> read/write for example).
          """
          if xer not in primary:
              primary[xer] = TwistedSocketNotifier(None, self, xer, type)