US Attorney-General William Barr's summary of the Robert Mueller report is only a first step in the accounting of President Donald Trump's behaviour, but it is not too early to reach this conclusion: It marked a good day for some norms - but a much, much worse one for others.

A special counsel's investigation of a president is first and foremost a mechanism for addressing strictly legal questions. But it is necessarily always more, and the investigative process and the ultimate legal and factual findings reflect, help shape and often reinforce the state of key norms - fundamental expectations of presidential conduct.