code: func (api *HandshakeAPI) SendSym(symkeyid string, topic Topic, msg hexutil.Bytes) (err error) {
  	err = api.ctrl.pss.SendSym(symkeyid, topic, msg[:])
  	if otherErr := api.ctrl.registerSymKeyUse(symkeyid); otherErr != nil {
  		return otherErr
  	}
  	return err
  }
docs: // Send symmetric message under the handshake scheme
  //
  // Overloads the pss.SendSym() API call, adding symmetric key usage count
  // for message expiry control