I'm posting this mostly for my own benefit; this is discussing a language feature I'm unfamiliar with.
What I find interesting is that just like some ideas are in the air and find their landing point, so too do some ideas just fail to find the right implementation.
So time to look up sum types (or disjoint unions or...).
"...language features are easily broken, mis-copied, forgotten or intentionally omitted due to the designer's pet beliefs."