def yield(dt, clock = nil)
      clock ||= $shreduler.clock
      $shreduler.shredule(Shred.current, clock.now + dt, clock)
      Shred.current.pause
    end
def yield(dt, clock = nil)
      clock ||= $shreduler.clock
      $shreduler.shredule(Shred.current, clock.now + dt, clock)
      Shred.current.pause
    end
yields the given amount of time on the global Shreduler, using the
 provided Clock if given