IMPROV, or impro as Paul Merton and his quick-witted chums have shortened it still further, is comedy improvised in the moment.
"It's just for tonight," says Merton, arriving on stage first, as much the lead wit as he is on Have I Got News For You. "It's never been done before; it'll never be done again."
In content, that is true; in format and chums, however, it is as regular as, say, BBC Radio 4's I'm Sorry I Haven't A Clue, with its pattern of interactive games that start off with audience comments and then bounce off each chum's reactions and flights of fancy. The full-house audience is quick and eager to offer suggestions - surreal, risqué, bizarre, inspired, straight, daft - and York is in that weekend mood that always helps a gig go well.
Merton, as sharp as his jeans and baggy shirt are not, sets the casual yet on-the-ball tone, picked up by Lee Simpson, the most physically adroit performer; Richard Vranch, the keyboard player, guitarist and laidback cheeky chap; and Suki Webster, irresistibly saucy, savvy and more devastating in her impact for choosing her moments when to chip in.
The wild card is the big American tornado Mike McShane, more erratic but bursting with energy. From strimming Matthew Kelly's nasal hair, to "larging it" in a new Shakespeare drama, The Duchess Of Manchester, Merton and co can make merriment from anything - and then it's gone, as quickly as it came. Flashing wit indeed.