SCE — Some SCE Elements are concrete instead of abstract

During resolutions of SCE-7 and SCE-96, the three elements RootElement, KindSet, and Kind have been accidentally set as concrete but should have been abstract. In general, all SCE elements should be abstract and then specialized by concrete elements defined by the languages that are based on SCE. Some exceptions to this include concrete DI elements and the Model Artifacts (e.g., Group), which can be directly used by other languages without specializations. But that is not the case for the three classes mentioned before.

In addition, the XSD complex types tModelArtifact, tBaseElement, tElementType, tTypedElement, and tAnnotation are not abstract although their corresponding element definitions and metamodel classes are abstract.