code: def remove_prefix(self, prefix):
          
          if prefix not in self.__prefix_map:
              return
  
          ni = self.__lookup_prefix(prefix)
          ni.prefixes.discard(prefix)
          del self.__prefix_map[prefix]
  
          # If we removed the preferred prefix, find a new one.
          if ni.preferred_prefix == prefix:
              ni.preferred_prefix = next(iter(ni.prefixes), None)
docs: Removes prefix from this set.  This is a no-op if the prefix
          doesn't exist in it.