code: func (h *sentPacketHistory) FirstOutstanding() *Packet {
  	if h.firstOutstanding == nil {
  		return nil
  	}
  	return &h.firstOutstanding.Value
  }
docs: // FirstOutStanding returns the first outstanding packet.
  // It must not be modified (e.g. retransmitted).
  // Use DequeueFirstPacketForRetransmission() to retransmit it.