Update: Hum, I guess he didn't use the word "set", but "difference" is an operation that produces a set.

I personally believe that a notion of difference can be given for the mathematical objects representing both ordered lists, i.e. functions from some finite set (0...n-1) into some other set, and unordered ones, i.e. classes of equivalence of those functions wrt composition with a permutation on the domain. Except that at least two reasonable definitions of such a difference spring to mind, abstractly, and I think that with no further specification one just has to pick the one that seems most reasonable for the actual problem. In the example the OP gave, both would do, so there's an ambiguity.

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In reply to Re^5: Subtracting Lists by blazar
in thread Subtracting Lists by JimSki

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