code: def add(self, connection):
          '''Add a connection'''
          key = (connection.host, connection.port)
          with self._lock:
              if key not in self._connections:
                  self._connections[key] = connection
                  self.added(connection)
                  return connection
              else:
                  return Nonf add(self, connection):
          '''Add a connection'''
          key = (connection.host, connection.port)
          with self._lock:
              if key not in self._connections:
                  self._connections[key] = connection
                  self.added(connection)
                  return connection
              else:
                  return None
docs: Add a connection