private void SubscribeButton_Click(object sender, EventArgs e)
        {
            if (current_reactor != null)
            {
                // set subscribtion status
                current_reactor.setSubscribedPS(!current_reactor.getSubscribedPS());

                // add or remove from observer list
                if (current_reactor.getSubscribedPS())
                {
                    to.addObserver(current_reactor);
                }
                else
                {
                    to.removeObserver(current_reactor);
                }

                // change button text
                refreshSubscribeButton();
            }
        }