def callWhenRunning(callable, *args, **kw):
    """
        Call a function when the reactor is running.

        If the reactor has not started, the callable will be scheduled
        to run when it does start. Otherwise, the callable will be invoked
        immediately.

        @param callable: the callable object to call later.

        @param args: the arguments to call it with.

        @param kw: the keyword arguments to call it with.

        @return: None if the callable was invoked, otherwise a system
                 event id for the scheduled call.
        """
