Spawns a callable on a new green thread, scheduled for *ms*
        milliseconds in the future::

            def echo(pipe, s):
                pipe.send(s)

            p = h.pipe()
            h.spawn_later(50, echo, p, 'hi')
            p.recv() # returns 'hi' after 50ms

def spawn_later(self, ms, f, *a):
        """
        Spawns a callable on a new green thread, scheduled for *ms*
        milliseconds in the future::

            def echo(pipe, s):
                pipe.send(s)

            p = h.pipe()
            h.spawn_later(50, echo, p, 'hi')
            p.recv() # returns 'hi' after 50ms
        """
        self.scheduled.add(ms, f, *a)