There's a lot of talk about how unclear it is what John Roberts has the power to do in the impeachment trial.
I've seen claims that he can call witness and claims that he can't; there are articles about how it's unclear whether he can break ties, like the vice president. There are probably a lot more areas of confusion as to what he can do.
However, the Constitution only really matters when it is interpreted.
Who can formally decide what the Chief Justice can and can't do in an impeachment trial? The courts? The Senate? Where are his powers defined?