The current class hierarchy assumes that all window function expressions have a removable and non-removable variety. And the current translation code expects that the variety of executor depends only on the bound type (lower unbounded vs. not and document vs range bounds).  This turns out not to be the case for roughly a third of our expressions, so these classes have the potential at runtime to throw exceptions, since they still have to implement buildAccumulatorOnly and buildRemovable. 

The proposal is to move the translation logic into the expression classes as Expression::translate, and leave buildAccumulatorOnly and buildRemovable as private methods only where they make sense.