code: func (p *Pss) SendSym(symkeyid string, topic Topic, msg []byte) error {
  	symkey, err := p.GetSymmetricKey(symkeyid)
  	if err != nil {
  		return fmt.Errorf("missing valid send symkey %s: %v", symkeyid, err)
  	}
  	psp, ok := p.getPeerSym(symkeyid, topic)
  	if !ok {
  		return fmt.Errorf("invalid topic '%s' for symkey '%s'", topic.String(), symkeyid)
  	}
  	return p.send(psp.address, topic, msg, false, symkey)
  }
docs: // Send a message using symmetric encryption
  //
  // Fails if the key id does not match any of the stored symmetric keys