code: public void pluginDisappears( PluginInterface pi ) {
  
  		// May happen if a plug-in could not be instantiated
  		// (iPojo uses proxies). In this case, it results in a NPE here.
  		if( pi == null ) {
  			this.logger.info( "An invalid plugin is removed." );
  		} else {
  			this.plugins.remove( pi );
  			this.logger.info( "Plugin '" + pi.getPluginName() + "' is not available anymore in Roboconf's agent." );
  		}
  
  		listPlugins();
  	}
docs: This method is invoked by iPojo every time a plug-in disappears.
  @param pi the disappearing plugin.