Add custom taggedt union type back.
Add test function with proof of concept for the fully type-safe content model disjunction constraints.
Re-write the Element abstractions once again to use the content model constraints with type-safe union types, since the problem with ambiguous implicits have been solved through implicit prioritization.
Bring back the Renderer logic that works for the latest abstraction approach for Element.
Update the test file to the latest API  style.
Temporarily comment out all the code in Operations because with the latest API approach is not yet clear how operations on Elements should be organized.