There’s been a lot of talk about what this week’s Supreme Court ruling does or doesn’t do with respect to former President Donald Trump. In my view, most of those analyses miss the point entirely. Here’s the most important single fact to know about the high court’s landmark decision: It sets forth a framework for determining which actions by a president — any president — are absolutely immune, presumptively immune or not immune at all. I have a unique perspective on the issue, since...