Thank you for your posts. Now I begin to understand the warning messages. But I thought $1 or $2 will not be defined at the same time,i.e. for any moment, only either of them is defined, the other is undefined. We can say sometime $1 is undefined, sometime $2 is undefined; but I still don't think that $2 will never be defined, since in this example, "\U$2" indeed change "and" to "AND". Is my understanding right? Thanks.

In reply to Re^4: why such an error happened? by lightoverhead
in thread why such an error happened? by lightoverhead

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