Yes, the second type could in theory be an internal DSL. However, as you probably know, I am not interested in those (for various reasons) and I didn’t have those in mind when I wrote this.

Regarding your second point: they are included in the third case, these are domain-specific structures (for technical domains). The two inner layers (behaviours, expressions) are empty. It’s a corner case, and it has been a very long time since I built one of those.