SMOF_ — Extended element semantics complicates common use case

Section 9.1.2.3 (Element, as extended), Semantics, first sentence, says reclassification is constrained by Compatibility elements, but this would mean an instances of a type cannot be reclassified as one of its subtypes without using compatibility, see AspectOf example in the previous issue. This is such a typical use case that it would be significant burden to require modelers to explicitly define compatibility for it.

Metaclasses will be considered compatible by default and without need for specific marking, unless explicitly marked disjoint. There will no longer be any Compatibility elements. This will also resolve issue 17152. The required model changes are addressed by the resolution to issue 17163, which becomes a prerequisite to this resolution