public void delete_U(String u){
		
		DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();  
	    Query query = new Query("Usuario");
		query.addFilter("Username", FilterOperator.EQUAL, u);
		PreparedQuery pq = datastore.prepare(query);
		Entity Usuario = pq.asSingleEntity();
	    datastore.delete(Usuario.getKey());
	}