What would be the colloquial yet correct form to refer to the king of the Holy Roman Empire in a conversation where it's not clear that you're talking about the HRE? I mean, you can say "Holy Roman Emperor", many people do it. "Holy Roman king" would be unusual and "German king" might need some clarification as well, same for "Roman king" or "king of the Romans". Would it be "king of the Holy Roman Empire"?