I looked at all of the replies, but none of them mentioned the basic difference between a court proceeding (with lots of legal precedent) and a congressional impeachment (where they decide upon the rules they'll use, and are not bound by the Federal Rules of Civil Procedure). (I'm trying very hard to keep this comment non-partisan. I think I did it.)
The concrete thing I can't find anywhere is the actual, on-paper set of rules this people set up, ideally grafted to some kind of explanation on why this ruleset made any sense in context.
