code: func (self *Queue) DelayedTime(dur time.Duration) *QueueCursor {
  	cursor := self.newCursor()
  	cursor.init = func(buf *pktque.Buf, videoidx int) pktque.BufPos {
  		i := buf.Tail - 1
  		if buf.IsValidPos(i) {
  			end := buf.Get(i)
  			for buf.IsValidPos(i) {
  				if end.Time-buf.Get(i).Time > dur {
  					break
  				}
  				i--
  			}
  		}
  		return i
  	}
  	return cursor
  }
docs: // Create cursor position at specific time in buffered packets.