`SCDiff` has a reference to `SCThingsWithoutDiff`

What's wrong?

SCDiff has a reference to SCThingsWithoutDiff. The tests should fail in this case!

How should it be instead?

Any SCThing should have references to other SCTHingWithReferences and the tests should recognize this.

Why should it be that way?

The tests should work correctly, so that modelling mistakes don't happen, when coding.

cc: @krlwlfrt