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This adds a lot of flexibility to algorithms, as it makes output sinks truely optional. For instance, the various "Extract by..." algorithms could add a new optional sink for features which 'fail' the extraction criteria. This effectively allows these algorithms to become feature 'routers', directing features onto other parts of a model depending on whether they pass or fail the test. But in this situation we don't always care about these failing features, and we don't want to force them to always be fetched from the provider. By making the outputs truely optional, the algorithm can tweak its logic to either fetch all features and send them to the correct output, or only fetch matching features from the provider in the first place (a big speed boost).