I followed this tutorial: https://www.youtube.com/watch?v=xXmYE9HrooM
I also tried other tutorials but it all comes down to:
I create a concept, set "instance can be root" to true,
try to create an instance in the sandbox solution,
but "New" only contains "Test Model"...
I have no idea what I am doing wrong here.
When adding baseLanguage as a Language to the sandbox,
I am able to instantiate the root concepts of the base lang, but still not mine.
As this might be relevant: I am using the current MPS EAP (3.4)
Thanks for your help