def lookupSubsystem(subsystem, data):
        """
        The other side requested a subsystem.

        We return a L{Protocol} implementing the requested subsystem.
        If the subsystem is not available, we return C{None}.

        @param subsystem: The name of the subsystem being requested
        @type subsystem: L{bytes}
        @param data:     Additional request data (often nothing)
        @type data:      L{bytes}
        @rtype:          L{Protocol} or L{None}
        """

        The other side requested a subsystem.

        We return a L{Protocol} implementing the requested subsystem.
        If the subsystem is not available, we return C{None}.

        @param subsystem: The name of the subsystem being requested
        @type subsystem: L{bytes}
        @param data:     Additional request data (often nothing)
        @type data:      L{bytes}
        @rtype:          L{Protocol} or L{None}
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/conch/interfaces.py