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the tree is like a key-value database, with the key being a domain name of a record which is an additional for a record in the zone while the name might be not in the zone; the value is a list_t of pointers to zone nodes, for which it actually is an addiitonal whenever in an incremental update a record is changed/added with the name of an additional, all the listed nodes must be re-adjusted (re-discovered additionals previously, we always re-discovered additionals for whole zone
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