public static String isCommand(String exp){
		if(comSet.size()==0){//
			for(int i=0;i<commands.length;++i){
				comSet.add(commands[i]);
			}
		}
		exp=exp.trim();		//ȥߵĿհ
		if(comSet.contains(exp)){
			return exp;
		}
		return null;
	}