code: func (c *Client) Logout(sessionID string) error {
  	payload := &Request{
  		Action: "logout",
  		Param: &LogoutRequest{
  			CustomerNumber:  c.customerNumber,
  			APIKey:          c.apiKey,
  			APISessionID:    sessionID,
  			ClientRequestID: "",
  		},
  	}
  
  	err := c.doRequest(payload, nil)
  	if err != nil {
  		return fmt.Errorf("logout error: %v", err)
  	}
  
  	return nil
  }
docs: // Logout performs the logout with the supplied sessionID as specified by the netcup WSDL
  // https://ccp.netcup.net/run/webservice/servers/endpoint.php