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**Features and Improvements**
- add ``Constant2.BackAssign`` method, define entity mapping in a more clean way.
- add ``Constant2.ToIds``, ``Constant2.to_ids``, ``Constant2.ToIds``, ``Constant2.ToIds``, ``Constant2.ToClasses``, ``Constant2.to_instances``, ``Constant2.SubIds``, ``Constant2.sub_ids``.
**Minor Improvements**
- from 98% to 99% test converage.