Protect adds a given Ref, and all Refs reachable from it, to a Keep.

Run runs a garbage collection on store, deleting all refs not protected by k.

ProtectPair is the type of an item returned by a ProtectFunc.

Store is a DeleterStore that delegates calls to a nested DeleterStore and can count refs and deletions during a call to Run.

Keep is a set of refs to protect from garbage collection.

ProtectFunc is the type of the callback passed to Protect.