throw new NoPushPopException();
throw new NoPushPopException(this.toString() + " did not push or pop anything.");
    public void cleanupWork() {	// if we have not pushed or popped anything, throw exception.	// Do not count this as an execution.	if (currentPopped==0 && currentPushed==0) {	    throw new NoPushPopException();	}	// for phased filters, executions are incremented at phase boundaries	if (!(this instanceof PhasedFilter)) {	    numExecutions++;	}    }