On sketch shows, the wisdom once was that you needed a punchline. That is, a slightly hammy, summative sign-off to let people know that they had come to the end of any given bit, to help the audience keep its bearings. The rules changed when the team behind Monty Python, who hated writing that mugging final joke, discovered that you could simply cut to Graham Chapman wearing a dress in a field and saying in a stern voice: ‘And now for something completely different’ — and it turned out that this was not only just as good, it was actually...