What's -mutableSetValueForKey: returning, actually? Is that something special?

From the docs:

Y开发者_StackOverflow中文版ou usually access to-many relationships using mutableSetValueForKey:, which returns a proxy object that both mutates the relationship and sends appropriate key-value observing notifications for you.

So this returns an "intelligent" NSMutableSet which automatically lets the context delete objects when they get deleted from the set, and reverse? Is that a proxy object?

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Yes. See Accessor Search Pattern for Unordered Collections in the Key-Value programming guide for details.