public boolean liberer(String num)
    {
    	boolean informer = libres.isEmpty() ;
    	boolean libere = reservees.containsKey(num) ;
        if (libere)
        {
        	Place place = reservees.remove(num) ;
        	place.setLibre(true) ;
        	libres.put(num,place) ;
       		if (informer) informer() ;
        }
        return libere ;
    }