But I think she has already told us that he is not [a horcrux] in Book 4.
I don't remember that. I don't think Harry's a horcrux, either, but I don't remember anything about it in book 4. Of course, I haven't reread GoF for a while, so there's a lot I've forgotten.
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Never let the facts stand in the way of a good answer. -Tom Magliozzi