see: https://github.com/xtuple/xtuple/pull/1275/files#r9780580

I see the basics of how everything is connected, even if I'm not completely enlightened. My point is that there are several functions in `ModelMixin` that specifically suit the requirements of `SimpleModel`, and so the implementation should simply be in `SimpleModel` so that `XM.Model` shouldn't have to override all this stuff that it would never have any intention of using. And moreover there appear to be differences between `XM.Model` and `XM.ModelMixin` which are unrelated to the requirements of `SimpleModel`. That's why I requested that `@override` be used to affirm the correctness of functions that look very much like copy/paste errors but which are in fact intentional.