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* some inserts to go to other equally-good subtrees.
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* keep_current_best is -1 if we haven't yet had to make a random choice
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* decided to keep it, keep_current_best is 1; if we've decided to
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* replace, keep_current_best is 0. (This state will be reset to -1 as
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* soon as we've made the replacement, but sometimes we make the choice in