code: func (r *Rx) Flush() {
  	if r.head == r.tail {
  		return
  	}
  	r.p.writeAtomic(r.tail, slotFree|slotToPayloadSize(r.head-r.tail))
  	r.tail = r.head
  }
docs: // Flush tells the transmitter that all buffers pulled since the last Flush()
  // have been used, so the transmitter is free to used their slots for further
  // transmission.