SBE's yield policy class PlanYieldPolicySBE is a child of PlanYieldPolicy, and it appears that throughout the SBE engine, all yield policies are the concrete class PlanYieldPolicySBE, but nearly all the variables used to hold and point to these are of the parent class PlanYieldPolicy. This reduces readability of the code and in fact hides from the reader that the PlanYieldPolicySBE class even exists. (I didn't discover this until many weeks of working on a yield-related project.)

We should replace PlanYieldPolicy with PlanYieldPolicySBE throughout SBE to clarify the code.