Keep turning over in my mind something I just heard on the excellent NPR Politics podcast.
If you'd suggested in the first year of George W. Bush's second term that the next President would be Barack Obama you would have been laughed at.
And if you had suggested in the first year of Obama's second term that the next President was going to be Donald Trump the laughs would have been even louder.
I don't know exactly what it proves other than the enduring truth of Harold Macmillan's line about the thing politicians most fear.
"Events, dear boy, events."