When we acquire the lock to node 2, we can unlock the lock on parent node 1, because at this point, it is safe to assume that any future operations will not modify node 1 (because we intend to modify node 3 or 4, which only potentially affects their parents).
When we acquire the lock to node 2, we can unlock the lock on parent node 1, because at this point, it is safe to assume that any future operations will not modify node 1 (because we intend to modify node 3 or 4, which only potentially affects their parents).
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