Suppose a witness W is asked a question on cross examination and W testifies p as to what happened between W and another person Y.  Perhaps W is a party.

Now suppose the the cross examiner “Y is probably going to come here and testify, you are saying that if Y says not-p he will be lying. True?”

So at this point what should W say? He has several choices:

(1) “Yes he would be lying.” The chances are this answer should be avoided.

(2) “I don’t know.” Again, this unequivocal answer should be avoided if possible.  This statement may be true, but there is at least one other truthful answer that may be better. 

(3) “If X asserts not-p, his statement will be false. Maybe he will be merely mistaken Whether he is lying or not is a different matter. I can’t know for certain what’s going on in his head.” 

I suggest that (3) is the truthful answer W should probably give to the cross examiner’s question. S/he is almost certainly simply trying to score a rhetorical point with the jury .