The suggested default Iterable that should be emitted when data is missing or unexpected in some way.
The suggested default Iterable that should be emitted when data is missing or unexpected in some way. This is provided to the feature generating functions but not doesn't act as a substitute for the framework's responsibility for handling missing data.
Note: The keys (1st field) present in the Iterable's Tuple2s should be prefixed with "=". This is because (as the name suggests, it is mainly for regression models which have a specific format for generated features.