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The problem with this assumption is that after doing inexact mathematics of more than a few steps, you run the risk of getting accumulated errors. When you have accumulated errors from several roundoffs, you get an approximate answer, and you *CANNOT* claim that there is no representable inexact number closer to the correct answer; after as little as four inexact operations, it is unlikely to be true. The answers in inexact math, other than trivially simple single operations, are not "the closest representable inexact number" to the answer, which in your opinion is the key feature that transforms them into neighborhoods; they are simply answers which are known to be wrong, but which we hope are reasonably close. Bear