Since the election of United States President Donald Trump, commentators have rummaged through the past to make sense of the present and predictions about the future. They are preoccupied, in a word, by precedent.
At the heart of any precedent is the belief that once acknowledged, it is actionable. A historical precedent offers not only a pattern but also a promise. We are assured by a rule - given what has preceded, here is how we must proceed - and reassured by a pledge: If we act rightly, we will be around to act again.