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Root interface for filter callbacks.
Filter callbacks encapsulate two ideas : 1) that we filter for
some object in the space of cluster objects and 2) we want to be logger.informed
when new/changed/deleted things match the filter.
There are generally listener interfaces for every callback.
Very simple callbacks may not have an explicit listener class
paired with it. And some callback classes share a listener class.
The interface doesn't require a listener, but the adapter does.
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