John Cage anecdote

From the Guardian's The music of chance article, in which various people reminisce about John Cage, Stephen Montague says in part:

We were at the Opéra Bastille in Paris, and the building still wasn't quite finished. There was a party on, and John and I, along with, Merce Cunningham, Mme Duchamp (Marcel Duchamp's wife) and the composer Yvar Mikhashoff, got into a service lift to try to find it. But the lift stopped between floors, and the lights went off - we were plunged into total darkness. It was plain weird. Every second I expected the lift to go into freefall. We pressed the emergency button, and waited. Nothing happened. Finally Mme Duchamp said: 'What are we going to do?'
John replied: 'It's the perfect opportunity to hear a piece of music. Just listen.' There was a sort of rumble, a kind of hum from the building. We all listened intently. After a while, Yvar started some irregular, very occasional tapping, and so did I. Finally - after about 20 minutes, though it seemed like hours - the lights went back on and we were able to get out. Later, John said: 'Wasn't that a marvellous piece of music. My only sadness is that two people were adding dissonances to it.'