code: func (nc *Conn) newRespInbox() string {
  	if nc.respMap == nil {
  		nc.initNewResp()
  	}
  	var b [respInboxPrefixLen + replySuffixLen]byte
  	pres := b[:respInboxPrefixLen]
  	copy(pres, nc.respSub)
  	rn := nc.respRand.Int63()
  	for i, l := respInboxPrefixLen, rn; i < len(b); i++ {
  		b[i] = rdigits[l%base]
  		l /= base
  	}
  	return string(b[:])
  }
docs: // newRespInbox creates a new literal response subject
  // that will trigger the mux subscription handler.
  // Lock should be held.