Technically, not "not model-able" but "not modeled". As in, the effort was not done, and is easy to omit. And doing it in the general case is a lot of work, hen

Technically, not "not model-able" but "not modeled". As in, the effort was not done, and is easy to omit. And doing it in the general case is a lot of work, hence the expand-contract and only-two-versions designs.